For SF & F this seems to be a deliberate choice, from what I understand. This is from an iterview with Ted Chiang:

TC: I think the reaction varies, because science fiction is a more commercial genre. There are a lot more people in science fiction whose goal is to make a living from writing fiction by publishing one or more novels a year. And people who enter science fiction generally receive more messaging about fiction writing as a sole source of income than, say, people entering mainstream fiction. The messaging there is different: get an MFA, teach; it’s understood that your teaching position supports your career as a writer. For writers entering science fiction, that’s not really a thing yet. We’re maybe getting there, but the messaging they receive is mostly: Be very prolific.

https://culture.org/an-interview-with-ted-chiang/

>> As [Stephen King] got more and more famous, he started being able to bully his editors, to the point where his work is barely edited at all.

Yeah, I kinda noticed that too. I picked up a book of his and couldn't finish it because it felt like every episode in the story was written so that it took maybe ten, maybe fifty more pages than it really needed to. Extreme padding.

I think Liu Cixin also went through something similar. The Dark Forest was lean and mean. The next two books were progressively fatter and more verbose and full of aimless meandering. Though maybe that was an attempt to complete a trilogy to capitalise on the first book, if I'm more cynical.