That isn't guaranteed, though. The other day I wanted to create a new community and was browsing instances on join-lemmy.org/instances for an instance that was compatible rulewise. The one I picked evidently wasn't a good pick (burggit.moe). Trying to advertise my new community, I found out it was defederated from beehaw (and likely others) and got insulted as a pedophilia sympathizer ...

Randomly assigning new users to instances would make a substantial fraction of people very unhappy.