To use coding terminology, there's a major distinction between "refactoring" vs "rewriting". Refactoring involves many incremental and iterative changes to an existing system. Whereas rewriting is discarding all incremental changes, and instead building a new system from scratch.
Refactoring is generally the most reliable way to improve a system. But rewriting is the best way to get promoted, and do work that feels personally "fun" and "impactful". I suspect this is the decisive factor in a large fraction of UI overhauls
