I get the idea of instances, like you can make your own and this is good for privacy. But some lemmy instances are much more popular and this in fact makes it another Reddit. If there are separate instances for niche topics, why not make it another community inside a larger instance?
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.