Sure, and I use Tuta. Those are outliers, the vast majority use gmail, or at least the vast majority in my circles do.

It’s the same thing as the network effect, just a little less ubiquitous, people will tend to use whatever everyone else uses. Getting something new like email (SMTP) is a huge endeavor, it’s a lot easier to just build a centralized service and get people to use that, and most people will use the same provider anyway.

I don’t like it, but I understand why it works and is so common.