I think there's a few factors, codebase size is one, and the tendency for vibe coding to be mostly additive certainly doesn't help with that.

But vibe coding also tends to produce somewhat poor architecture, lots of redundant and intermingled bits that should be refactored. I think the model is worse the worse code it has to work with, which I presume is only in part because it's fundamentally harder to work with bad code, but also in part because its context is filled with bad code.