Of course I can't speak for everyone else, but: I've been asked to become a mod for a sub with 2.3 million users, and I have contributed to that site for almost a decade. It took me 3 days to save and then manually delete all of my posts, and I'm still working through the comments a week later. It absolutely HAD to be done, because I'm not going to let a certain someone earn cash with my literal years of unpaid volunteer work any longer, but I would be lying if I said that it was an easy decision.
Why? Because that action punishes the users. A whole lot of what I posted were in-depth game guides, and reddit users now no longer have access to those. I regularily called out scammers, provided sources for artworks, answered dozens of questions daily - I felt responsible for that sub and its users. And if you feel responsible for something, then you can not easily toss it away without feeling a certain degree of guilt, whether that feeling is justified or not.
But just for the record: I do not regret the decision. Yes I feel a bit bad for the community, but it had to be done. I can still understand why others might be more reluctant tho.
(and of course there are also power mods who just don't like losing their status / influence, but that's a different story)