To me, it starts to feel like work when I have to rebuild large parts of my factory because I didn't leave enough space to expand. This process feels exactly like having to refactor a bad codebase (where you feel the urge to just start from scratch).
If you plan your build properly, you can avoid this, but it takes a few runs to learn the best strategies.
Factorio makes me realize that I don’t really like solving problems for the sake of solving problems (or optimization, for that matter). I like solving them for a tangible reason or because I I’m making someone’s life better.
Problem solving and optimizing in Factorio is different than a grind in some game like Diablo. That sort of grind uses a completely different part of your brain (movement and in-the-moment decision making), and I find that I need that kind of change after a week of real-life problem solving.
I imagine people who gravitate to Factorio are probably better problem solvers and optimizers than me because that’s what they truly enjoy for its own sake.
I think the new expansion helps a lot with the rebuild issue, since you're expected to have have many different bases on different planets instead of just one big base.
If you plan your build properly, you can avoid this, but it takes a few runs to learn the best strategies.