1) When I already have a rough picture of the solution to some programming task in my head up front, I do not particularly look forward to actually going and doing it. I've done enough programming that many things feel like a variation on something I've done before. Sometimes the task is its own reward because there is a sufficiently hard and novel puzzle to solve. Mostly it is not and it's just a matter of putting in the time. Having Claude do most of the work is perfect in those cases. I don't think this is particularly anything to do with working on a ball of mud: it applies to most kinds of work on clean well-architected projects as well.
2) I have a restless mind and I just don't find doing something that interesting anymore once I have more or less mastered it. I'd prefer to be learning some new field (currently, LLMs) rather than spending a lot of time doing something I already know how to do. This is a matter of temperament: there is nothing wrong with being content in doing a job you've mastered. It's just not me.