That's because it's been around for quite a while, and for a long time it was the best webmail service. It's also free, unlike most alternatives. And switching to a new provider means a new email address, unless you're using a custom domain with Google Workspace (or whatever they call it these days), which is a small minority of personal accounts.
(I gave up on Gmail a few years ago and switched to Fastmail, and like it much more than Gmail. But I'm the rare person who is willing to pay for email, and had been using a custom domain with Gmail, so my non-monetary switching costs were minimal.)