Also us millenials and zoomers are just more comfortable building on pre-existing work in general. Probably because a lot of us grew up playing out imaginary episodes with action figures on guest room beds or writing fan fiction online.
At the risk of being the token senior citizen . . . trust me, we did the same thing back in the day. It's just that the fan-fiction was on paper, not on-line, and our plastic toys were maybe less snazzy. (Google "Captain Action" and "Major Matt Mason" and Star Trek Colorforms.)
Just to belabor the point, there is this odd notion out there, which I've encountered before, that fan-fiction is a new thing that was born of the internet, which is very much not the case. Trekkies were sharing their mimeographed fanfic zines (many of the them featuring K/S) at conventions and via the mail as far back as the early seventies at least. And I can personally attest to filling many a spiral notebook with my own handwritten fanfic at least as far back as 1975 . . . .
Some years ago, I actually stumbled on a long-forgotten stash of my old teenage fanfic while clearing out my parents' old house. Weird to read that stuff again, after fifty-some years!