They do, but what I think they might not understand is that there is a protocol behind it all that at least theoretically allows for anyone to run their own server without a third party, which I think is the aspect of e-mail that you're trying to call attention to, no?
Part of the point that I'm trying to make is that e-mail as it stands today - with the majority of messages going through the servers of a single corporation - isn't all that decentralized in practice.
When you mention e-mail to the layperson without additional explanation, what they have in mind is a fairly centralized system.
To be fair, Matrix as it stands today isn't very decentralized either. The dev team reports that their big matrix.org server has something like 40k monthly users. And while there are a ton of tiny little personal homeservers out there, all the action is really happening on matrix.org.
