People are interested in communities and content. The idea of using one single client and being able to access all types of content from any kind of instance sounds great, but it's still a dream. Let's say that I am interested in memes and the communities I know of are most active on lemmy.world. Realistically, with my Mastodon account I am not going to have the same experience that I would have with using Lemmy just for that. I need to register a new account on lemmy.world.