In hopes that you really are a loser and will run straight into 'em
I assume that the "levels" aren't actually made by Bowser, but rather, he is simply making use of them, in an effort to protect the keys. **The same question can be asked of why Bowser fights in places designed like the ones in the original Super Mario Bros, where he gets dunked into lava, with a similar answer - he didn't make those levels, he just utilised them.
You'd think after the second time, he'd at least choose a different type of venue...
"A gentle sea dragon lives here. Pound on his back to make him lower his head. Don't become his lunch" (talking about Dorrie). What's this about becoming Dorrie's lunch?
Supposedly, you could slip off of Dorrie's head before he raised it, resulting in Mario landing in front of his mouth and getting eaten accidentally. As far as this troper knows, this wasn't actually possible, and was either just a joke, a rumor, or perhaps a small feature that got scrapped in the final game.
Seemed to be a scrapped feature, which made it more obnoxious by the Narm Charm applied when Dorrie looked more Yoshi-like in the DS remake.
Are the painting worlds entirely created by Bowser, or are they links to places that already exist?
Considering the Throwback Galaxy, I'm inclined to believe it's the second option.
This troper thinks of the courses as the paintings themselves, and Mario as an intrepid fictioneer. It would explain how he is able to jump between different points in time within the painting world, apparently temporarily replacing the past version of him who originally obtained that particular power star, but how it will just be an outline and history will only be rewitten if he succeeds.
Given the ? Blocks everywhere in the Mushroom Kingdom, it probably so they could blend in with the environment more easily.
Why did Bowser leave the door to Bob-omb Battlefield unlocked? If it wasn't for that level, you wouldn't be able to get any stars.
Anti-Frustration Feature, maybe? In-universe, probably this is part of the effort the Toads are doing to save the Princess, they were able to open that door, and we can do the rest.
Or what if—work with me here—what if Bowser used some sort of consumable resource to lock the doors? Let's call it Anti-Star Power. He started by locking the door leading to Bowser in the Sky, and in order to provide powerful protection to his final stronghold, he blew 70 Anti-Stars on it. He used 50 Anti-Stars to lock the door to the castle tower as an added layer of protection, and 30 to lock his secondary stronghold in the basement. By this point he was running severely low on Anti-Stars, so started doling them out in single-digit quantities to the remaining accessible doors. He miscalculated and ran out before he could lock Bob-Omb Battlefield. He also didn't realize that Mario would retain possession of the Power Stars after using them to unlock doors, or he would have been a lot smarter about distributing the Anti-Stars.
How did Koopa the Quick get from Bob-omb Battlefield to Tiny-Huge Island?