Because it’s not super easy. DNS management isn’t a button-click process, for the most part: if you’re trying to control your data and care about privacy, there’s no super easy service out there for you. You need to find a domain registrar, buy the domain you want, and configure it yourself. The help your email provider gives you to configure it might be grammatically different from how your registrar wants it entered, so it can be an annoying process.

It’s not rocket surgery, but it’s not just “enter name and password and IMAP server name and voila” either.