Although the way the game acts about it, it seems Zombies are a common nuisance and commonly dealt with by a company that apparently monopolizes the anti-zombie plant market, escaping would just be impractical. Besides, I would have a pink tricycle...
Some Fridge Horror to go with this. You know in 4-10, the zombies imitate your mother and request you to come over to her house? How would they eat your brains if you're not at your home? This can lead to two possibilities, both horrifying.
They turned your mother into a zombie
They're going for the other members of your family.
Considering the same note asks you to leave your lawn unguarded and your door unlocked, there has to be someone else in the house that they're after, otherwise they wouldn't bother with that request.
Think about the concept of the game: You're fighting zombies, living corpses, with plants and fungi — like decomposition! The zombies are a perversion of the natural order, and by creating the plant towers to destroy them, the player brings said natural order back.
Why do you think the Kernel-Pult can lob butter? Vegetable fat. Nuth said... Either that or because corn tends to be eaten with butter quite often.
Crazy Dave isn't wearing a pot on his head because he's crazy; he's protecting his sweet, delicious brain from being eaten by zombies. This may be cause for an Oh, Crap! moment later — right before the Final Boss, a bungie zombie grabs Crazy Dave, and the pot falls off.
Disco in PvZ seems to be a Popularity Polynomial, considering that it's become popular again in the future as seen by the Disco-Tron 3000.
Just came to realise this: zombies lose their left arm (or right on certain occasions…) when they take enough damage. The left arm usually hurts first whenever a person starts to have a heart-attack. Coincidence?
The Jester Zombie can reflect almost all projectiles... except the Magnifying Grass' sunray shots. It's pretty impossible to Catch and Return a projectile that isn't physical. Despite this, it can reflect Citron's plasma balls but not the Plant Food powered version.
Despite the Excavator Zombie's shovel clearly being a metal object, it can't be stolen by the Magnet-Shroom. It's made of gold, which is non-magnetic.
Why isn't the Phat Beet affected by Boombox Zombie's boombox music? He's wearing headphones, so he can't hear the music from the boombox.
It seems odd that the Breakdancer Zombie shares a Rap Jam Theme Music Power-Up with MC Zom-B, being the only zombies to share a preferred Jam. Not quite odd as Breakdancing is usually danced to rap music. Furthermore, Rap music videos tend to have a rapper (MC Zom-B) and dancing performers (Breakdancer).
Why do the dinosaurs only react to zombies and not the plants? They're used to seeing plants, as there were LOADS of plants around the Mesozoic Era. Zombies on the other hand, yet alone bipedal humanoids, never existed, so of course the dinos would be terrified seeing such a strange race.
In Jurassic Marsh, why can't the plants hurt the dinosaurs, yet can hurt the zombies? Because altering something from the past will cause a Butterfly Effect on the future. Jurassic Marsh Zombies on the other hand don't belong in the time period, so it's ok to kill them without altering the course of history.
Why does Dusk Lobber have the ability to fire three exploding buds at a go when powered up by Moonflower? It's based on a Prickly Pear Cactus, which are known for growing multiple buds on each "pad".
In the Bible, sheep were associated with good and goats were associated with evil. When hit by Wizard Zombie's and Rose's Baleful Polymorph spells respectively, the plants (who are the good guys) get turned into sheep, and the zombies (who are the bad guys) get turned into goats.
Partially a byproduct of re-used sound effects, a plant-munching sound can be heard when the zombies finally reach your house. However, this doubles as the sound of your skull getting munched by the zombies. You hear the Big "NO!" AFTER that sound.
As mentioned above, it is possible there is a young child in the house. What if the Big "NO!" is you seeing the zombie eat your child?
If you scroll down on the Achievements screen, you'll find a lot of ShoutOuts to other Pop Cap titles buried in the dirt ... but go far enough, and you'll find China, also overrun by zombies. The game's bright colors and tongue-in-cheek style work to hide the fact that the entire Earth has succumbed to the Zombie Apocalypse. That's right, the world has ended and you didn't even stop to think about it.
When the zombies drop seeds, you don't even think about it. You just figure that they picked it up somewhere. But once you remember zombies recruit, that's when you realize, SOMEBODY HAS DONE THIS BEFORE. Lots of people have! The zombies carrying seeds were once fellow gardeners, whose brains have been eaten. You are just retrying what hundreds have done before you... and they all failed.
On another note, the zombies could have been alive for a while. And during that time, everyone could have been turned into a zombie. EVERYONE BUT THE PLAYER! But hey, I'm not ruining your fun.
Anyone pay attention to Dave's dialogue before the battle against the Zombot Tomorrow-tron when Zomboss activates his hypnosis? Because it sounds eerily similar to the dialogue at the end of the tutorial...
Crazy Dave finally gets his taco back, albeit with a waffle on it. One wonders if Dave ate the taco and then he wanted to eat it again, considering he had broken the time-space continuum...
How on the world can a MASSIVE ZOMBIE ROBOT STAND ON A ROOFTOP WITHOUT IT COLLAPSING!?!?!
How on Earth can said massive zombie robot constantly slam Winnebagos down on the roof without it collapsing?
How on earth can plants resist a zombie invasion? Rule of cool.
Tingle's Law; Anything the developers do not want to break will not break, no matter how many times you hit it with your sword.
When a headwear zombie steps on Spikeweeds/Spikerocks, their headwear takes damage instead of the zombie itself. Despite their feet being stabbed by the spikes. So next time you have to tread barefoot on gravel, nails, burning coal, or broken glass, wear a helmet on your head?
This one gets even more puzzling when you consider that zombies that carry shields in front of them (the Screen Door Zombie, the Ladder Zombie, and the Newspaper Zombie) take the damage instead of their shields taking damage when they walk over spikeweeds and spikerocks (they also take damage directly from any variety of lobbed-shot plants). How does headgear block damage from all directions when shields only block frontal damage?
To kill a zombie, the head must be destroyed or cut off; that is why the headgear takes the damage.
How on earth did Gargantuar climb on the roof in the first place?
Uh, there was a ladder… Either that, or Zomboss drops them off…
How did any zombies get on the roof? They've already been repelled from both yards.
You've repelled them from the front and back, but not from the side.
How does a mighty Gargantuar get into your front door... or climb over your backyard fence... ...or go down your chimney......
How can you actually water a plant that's on fire?
Lampshaded in the sequel's Zen Garden. When you water the Torchwood (or other fire-based plants, it's fire disappears, it looks sad, then relights it. Although it would be easier to water it by the roots, as it's a tree…
The aquatic plants note Lily Pad and Tangle Kelp don't exactly look thrilled when you water them either...
It doesn't make sense that a zombie can pogo stick on spikes.
Why on earth are there modern explorer zombies in Ancient Egypt? Or robot bulls in the Wild West? Three words: Dr Edgar Zomboss. Think about it. He might also be the reason as to why there are zombies everywhere in time...
Dr. Zomboss has a gigantic brain, so why do none of the other zombies ever hunger for it? No Zombie Cannibals.
How does the Shadow-Shroom poison zombies, which are already dead and rotting to begin with? Even stranger is that it can also poison robots...