The “cleanup pass” never happens, though. It’ll just be new feature on top of new feature until it’s too large to refactor.

You're describing a problem that's plagued corporate software development for decades. You just get to the "unmaintainable ball of mud" stage faster now. Every few days I spent a while on codebase architecture improvements after landing a slew of features.

Except it does. It's not some law of physics. I've done exactly this on multiple projects, both personal and corporate.