That's not the case for some rapid-releasers, but for most, it seems like they bank all on the release and not building an evergreen shelf.
Yes. I've noticed it. I recently read a book by a writing coach, and I was shocked to see that in her mind, authors have to release every month or tank, with some few exceptions. She then explains that not everyone can release every month, of course, and advises writers to keep their day jobs. I don't know why I read that book.
Nothing against the author, of course, who is giving the best advice she can give based on her experience.
Anyway, she seems to know a lot of writers who absolutely have to release every month, and a lot of writers who suffer burnout because of doing that or trying to do that. Is it maybe a romance thing? Perhaps there's so much competition that readers only read new releases? I don't read romance, but I read romantic fantasy, and there are some evergreen books, books that keep a good ranking for a long time, and authors with good income and few books (based on author ranking on Amazon). So I don't know where that mentality comes from.
Ys, I guess holding a series to release it fast is a good idea, but releasing a book every month isn't healthy for most people.