Servers are expensive, either someone pays, or the instance closes.
As of now very few people are on the fediverse, so it's cheap.
But if we expect hundreds of millions of people to use it... expenses will skyrocket.
That's the problem: it does not make money.
The fediverse mean that the different instances can federate, but every instance after all still has its server and its expenses just like every web service since... always. Forums, chats, socials...
So far donations or pro bono from whoever is managing an instance, but unluckily this is not sustainable if user number skyrocket.
A way would be to somehow make everything p2p but I don't even know if it's possible for what is needed.