This is the reason behind essentially every reply I've ever seen to this question.
"I like it this way because it's always been this way and once you change your entire email workflow and customize your email client, it's almost as good as PHPbb"
Forums are built for threads and are immediately visible and accessible for everyone, not just people who want to spend their limited time dicking with email clients.
Mailing lists are the proto-discord: knowledge locked away from the public behind a special frontend and elitist attitudes. It's only better because the list is technically visible, but only in the worst, most low-effort way possible. You dump a raw txt copy of the entire thread unstructured onto the user and make it their problem to figure out. After all, your email client makes it easy to read, so why should you care about what anyone else needs?
I think it's unsurprising that the people maintaining the tangled plumbing of the clunky, customizable, fiercely independent operating system most popular with highly-opinionated power-users prefer the clunky, customizable, open format that's a little inconvenient for non power-users but allows them to set up their own personal bespoke client.
I understand it is out of fashion, but technologically more advanced are systems that use well defined interfaces and allow pieces to be exchanged easily. After all, we’re communicating over a number of open protocols here. A forum merges all elements into one silo. I prefer web over AOL/Compuserve. If you want a forum-like interface, there’s no technical reason why this couldn’t be done on top of a mailing list. In fact, Discourse and others attempted it.
This discussion has been happening since forever. And also the idea that it serves anyone to complain how others are obviously doing it wrong, without even attempting to understand why they’re doing it a certain way. And then be irritated when the response is negative, and labeling others as elitist for using and providing open platforms over decades and not silos.
If you don’t know, feel free to ask. And then suggest (or provide!) improvements that factor in current requirements and goals instead of dismissing them as stupid.
Life advice: if you want anybody to change what they do, you need to first understand why they’re doing it, and then offer suggestions based on that understanding that improve it with them. Otherwise you’re going to continue to recreate your own victim position, and an “elite” position that you will never belong to.