This used to be the case for me too, but hasn't now for years. I've followed plenty of people that migrated during the big exoduses from Twitter and then for a while started liberally following people whose reposts I liked and now the timeline is oozing with life ever since.

I don’t want to argue about the overall trend based on a single example, but Terence Tao’s substantial use of Mastodon for communication does change the picture a bit.

This is because you are holding it wrong. BlueSky and Twitter and alike are algorithmic attention grabbers. They try to feed you as much controversial stuff you might react to as possible. In the fediverse, there are no recommendation algorithms yet, so you need to make sure to connect with people you are interested in proactively.