I think the problem is not so much that "communities don't exist", but that they are far less popular and active than the lemmy.ml ones, and when presented with a choice new users will typically choose the community that is more active and has the most subs. You can't simply solve that by creating another community on another instance. A concerted effort would be needed to get people to move and to get them to pick the alternative community over the lemmy.ml one. Raising awareness and defederation by bigger instances (like lemmy.world) would help immensely.
For me the big ones are !linux and !programmerhumor btw, which do exist elsewhere but the alternatives are stale.