Not just an ego thing - some people have livelihoods that depend on their follower count. Twitter is very important to update Youtube subscribers, Twitch followers etc. on things. Remember, we are in the early stages of the Technological Adoption Life Cycle (assuming the Fediverse actually catches on) so most people don't use the Fediverse. As a content creator, you have to adapt to what people currently use.
So much for the "free market" lmao