Storage (/infrastructure) is only a small part of the GitHub/GitLab functionality; with a bare repos one solves only one problem of many :-)

Of the 2 employers I've had who self-hosted, they both did this. I've also done this when self hosting, it's super easy.

I guess there are probably no surveys of this though...

It's super easy for you. I tried to set this up but it's such a pain in the backside to tell people how to pull the latest code. Ended up with Gitlab and haven't looked back.

I didn't even realise Gitlab doesn't have auto-deploy yet.. We set up a .gitlab-ci.yml script that deploys the code.

For personal use, indeed, just use an ssh remote + gitweb interface with basic authentication and that's it.

For organizations it makes sense to have something more featureful.

For CI, you could have a git hook that could trigger a Jenkins build. For code review, you could use a tool like gerrit or phabricator or even set up an email list to send patch sets.

What other features do Gitlab, Github, or Bitbucket have that wouldn't be covered by that type of setup?

There are no bounds in engineering to how many wheels one can reinvent/reimplement. Ultimately, it's a matter of deciding where to invest the (limited) engineering resources.