Because email federation is inherent to everyone's understanding of how that service works. And perhaps more importantly, email "instances" are run by corporations. Laymen are not signing up on a "server" or "instance," they're signing up for Google, Apple, or Microsoft - the service they get aligns to a company that provides it. Nearly every single service that anyone has ever signed up for online has followed the same essential process: go to fixed url, create id and password, gain access.
It's easy to underestimate, especially in communities like this, how enigmatic the entire infrastructure of the internet is to the general population. Think of those videos where people are asked what "the cloud" is: they pause and ponder and then guess "satellites?" because they've never even wondered about it. I'm guessing that for many people, something like Twitter is just something that lives in their app store that they can choose to "enable" on their phone by installing it.
People know that software is "made up of code," but they don't understand what that means. The idea that an "application" is a collection of services run by code, that there are app servers and web servers, that there are backends and frontends, is completely unknown to (I'd guess) a significant majority of people. And if someone doesn't understand that, it's honestly near impossible to understand what anything in the fediverse is.
We need more, and more accessible, and friendlier, tech literacy in general. The complexity of our world is running away from us ("I have a foreboding [of a time...] when awesome technological powers are in the hands of a very few, and no one representing the public interest can even grasp the issues" - Carl Sagan) and we simply can't deeply understand many of the things that directly impact us. But because of its ubiquity, IT may be the best chance people have of getting better at understanding.