When someone replies to your comments, you get a notification. But clicking the notification takes you to the 'mentions' tab, instead of the 'replies' tab

unironically this.

You think 'oh it's not that hard to just pick a sever', but it is. Most people look at it and go 'well my favorite influencer or friend is on X, but I can only make an account on Y. Can I still communicate with them?! Which advantage has sever Y over server Z? etc. It's it's ONE barrier which is one barrier too much for many people (on top of all the new things they have to learn anyway when they decide to get on a new social network)

Most people don't know the ins and outs of how these federated systems work, like you do - and it's scary to them to be confronted with a question about system architecture, when all they want to do is read news or memes.

And it's interesting that you mention email, because I'd argue email has the exact same problem. Depending on which country you live in, you'll notice that most people use primarily one email provider per region/country. Why? because their friends use the same email provider. You know how many people told me "well, I don't have email, but I can give you my Gmail if you want...?" Email just 'took off' because it had nothing to compete with for 20 years and businesses depended on it as well.