How many businesses even store their internal code on GitHub's public service? Everywhere I've worked has used self-hosted solutions.

Every customer I worked with stores their code either on github or bitbucket. The only one who doesn't let me use my self hosted git repository (I mean the copy on a server of mine.) They also have a copy they can pull from there.

All of them are medium / small sized.

This doesn't work if the private repo belongs to your former company...

Once a company removes your access rights to their private repo, your historic commit record there disappears from your own commit timeline.

That doesn't help much. Our org, for example, runs private github instance. Only few things end up in github.com, and this is mostly boring stuff like patches to existing projects.