People do and have left communities in the past. /r/Marijuana to /r/trees comes immediately to mind and there have been many many others. But leaving for an entirely different service has a way higher executive cost. Once people are in the fediverse however, the cost to switching primary communities is not that high, and we've seen that away when people moved from !risa@startrek.website to !tenforward@lemmy.world due to mod actions.