1. Businesses need a "halp everything is on fire" button that costs them $1000 a second but will fix EVERY issue, (virtually) guaranteed by SLA. Ultimately that is what these orgs provide; the hosting is kind of almost secondary to that.

2. Actually building real-world implementations of "Issues, pull requests, comments, milestones, wiki, etc..." requires top-tier engineer[ing] time and energy to build and maintain. The current Git-on-steroids-aaS approach provides a very successful way to fund that undeniable cost.