This is the thing, it is much harder to replicate your network on mastodon.
Lemmy is easy, you just look for communities of interest and join them. It's largely a one sided affair (you the user), sure, people do have relationships in smaller communities but it's the exception rather than the norm.
Twitter I follow about 500-600 people, and have a shade over 4000 followers. This is a lot harder to rebuild on a new platform. And many of those connections are (online) relationships built up on the platform over many years.
Moving from Twitter is a bit like leaving school. You might maintain relationships with a few of your closest friends, but on the whole, you will not see most of those people again, however much you enjoyed your time with them