Reviews have become expensive, rewrites have become cheap

LLMs aren't lazy. They don't cut corners because a simpler solution feels good enough. If they know how to solve something thoroughly, they will.
An LLM defaults to building when it should be buying. Not because it doesn't know about existing libraries, it often mentions them, but because for an LLM, writing two hundred lines of implementation is the same cognitive effort as writing two lines of import. There's no instinct to reach for the shortest path. The shortest path for the model is to implement it completely.