I think it's partly because you're not gaining in gameplay terms. Narratively they're fine the first time through, but they are pretty long (Most notably the one in Kalm) and combine something you've already done a bunch of times with no progress for your characters, no new items or experience and stuff, and it ends up feeling like you're wasting time.

Kalm is literally the only part of the entire game I don't relish and look forward to.