I reckon a lot of re-writes tend to take "them 5x as long as they had hoped" and "isn't even any easier than the old code" exactly because writing the code wasn't the problem in the first place.

It's business logic, edge cases and other small necessary details that accumulated over time which make the code 'messy'. Once you've integrated all those in the new system, it likely looks equally messy. And discovering and implementing all those extra requirements is probably what took you the longest.

Not to say this applies to all re-writes or that AI tools can't help the process