People looking up alternatives doesn't mean they actually want to use them. Switching to an alternative necessarily means some pains, in terms of changed interface, functionality, and community. You can't just magically switch between platforms and have all the people follow you there, and that's a dealbreaker for many.

As far as I know, the fediverse is growing, just not explosively. We can hope that at some point it reaches a threshold that convinces a lot of people to move over, cementing it as a "proper" platform.