There’s no context to the question?

At face value, no, there’s no technical reason you can’t run a Plex server and a lemmy instance on one computer :)

“I made this…”

Actual answer? Who knows…

MY answer, because instance operator/server can establish whatever rules they want, and aren’t beholden to a CEO, shareholders, or any other entity that might want to put 50 ads on your feed for every post…

Because it’s open source and the community apparently wants it to stay that way.