Or they have better things to do vs fighting Route53 MX records errors.
The practical experience of having your own domain for your email is that you delegate your domain to Google / Fastmail / Proton / whatever, and it takes care of everything else. Some webmail providers will also let you buy a domain on their own website as a part of registration flow.
It really is not hard. Harder than not having a domain of your own, but not as hard as you make it sound.
It's just not something normal people do, but I don't like the snarkiness of implying that's an indicator of intelligence. Otherwise we go down the no true Scotsman rabbit hole, what do you mean you're using Proton. You didn't set up your own mail server ?
What do you mean you're using AWS, your not using a solar powered raspberry pi?
Plus all the good places to host your personal domain cost money, so now people who have gotten email for 'free' for 20+ years now have to start paying a monthly email hosting bill and annual domain registration fee because some 'nerd' told them they "should"? And if they ever forget to pay either one, suddenly their email is down and their email 'identity' is at risk of being resold.
This advice is exactly like changing your own oil: Anyone with enough interest in cars and dedication to learn the steps certainly can easily do it, yet nobody should try to convince their grandparents who aren't already highly self-motivated to start doing it.
If you don't trust Gmail, then you have to use Proton to host it, don't trust them then your on AWS, don't trust them you still need you ISP to play nice with a home server.
Unless you want to raise your own carrier pigeons...
Currently it's too hard for normal users, but it would be possible for e.g. Proton to add a feature where you can either import your domain name, or create a new one.
1. Most people will choose the free option, so it wouldn't be much of a useful revenue stream.
2. People having @gmail.com email addresses is a little bit of zero-cost marketing for them.