Because most people want to play without the hassle of setting up a private server and punching through NAT, or worse, CG-NAT. They want it to Just Work, and having the company run the servers is the best way to do that.

Ideally, they’d just set up the private server software for release and then just run some servers themselves, but I suppose it’s faster and cheaper to make the software just good enough for internal use than to polish and package it for public release, and have to support it too.