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...
Maybe it's because the homeowner is a collector of wheeled man-powered transportation?
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.
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. (Your Mileage May Vary)
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.
Actually, no, this is 2070 (maybe.)
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.
I thought they just attacked the Plant where the seeds come from. Which might be worse.
They drop seeds because Zomboss wants to annoy the player, so he takes the seeds and gives them to zombies.
Incidentally, it would also explain how there are zombies with plants for heads.
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.
Fridge Logic
How on the world can a MASSIVE ZOMBIE ROBOT CAN WITHSTAND 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.
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?
How on earth did Gargantuar climb on the roof in the first place?