It's kind of strange to me that the classic scifi books I read in my youth had few if any follow-ons, and in this case had to resort to other writers to happen.

It's kind of strange to me that the classic scifi books I read in my youth had few if any follow-ons, and in this case had to resort to other writers to happen. Meanwhile, many books I read nowadays on kindle routinely have 8 books in a series. I wonder what makes this happen? Is it that self-publishing that just spits things out with less friction? Less editing and/or second guessing? AI helping? Expectations?