In my experience it's mostly just the novelty of figuring out new starting conditions, and crafting a small system that you can know intimately without too much mental burden. We like figuring out challenges, but starting from scratch tends to be a greener pasture than building up what you already have.

It's an issue I have with all of these infinitely replayable games, and a fundamental part of human psychology, I think. Most people tend to start lots of personal side projects, but not many can truly commit to finishing something worthwhile.