Regarding:
"Why hasn't the fediverse grown more? Instances & immobile accounts!?"
That is part of it, I think.
But, (in addition to that) I suspect typical people want influencers.
But (big) influencers tend to make a living from their influencing activities. And if people don't want ads, then at the very least payments need to be in place (to get them to come here and stay here).
(I.e., the Fediverse would need payments to be part of the protocol.)