This is mostly because changing your email address is a huge pain. Once you're using your own domain, changing email providers is like changing web hosts or domain registrars: Some work but not a big deal.

Or, as an easier solution, a dedicated email forwarding service. I've been using one for about 15 years now and changed the actual email service provider a couple of times just by updating the forwarding address.

Of course then the problem is to identify a reliable email forwarding service that won't go anywhere.

what do you use for email forwarding

It's a non-profit for individuals in my home country specifically, doesn't target international audiences.