@tyil @ZekuZelalem I think the big problem is how Mastodon itself scales.
Almost every time that I hear that someone runs their own instance, they mention how resource-demanding it is, and how heavy it is to run one. Running an instance for others is an even bigger commitment, and there's no easy way to delegate a domain to an existing instance.
All this leads to making the most convenient thing: "few very large instances". People always go for convenience.