> Why pay someone to mess it up, when I can mess it up for free?
Because it isn't free. Your time is a huge cost.
A senior dev who spends even 10 hrs on standing up a git server has blown through years' worth of GitHub costs, and that's assuming you're even actually using the paid service.
Factor in the extreme security requirements of a code server, including needing to update dependencies daily, and you're spending far more time self-hosting with riskier results.
To add to this, and at the risk of restating my earlier points: even if you get it working, it still probably won't be as reliable as a provider like GitHub.
Keeping it secure is no small thing, especially if you want to permit access from arbitrary IPs on the Internet (rather than using a VPN, say). GitHub does this, and presumably they have solutions in place for everything from intrusion-detection to DDoS protection.
GitHub employs people to take care of server failover and data backups. You could spend your own time building your own solutions here, but they're unlikely to be as good as GitHub's. Your solution is guaranteed to be less well tested.
And that's assuming you even have a server room in the first place. You could run your own Git in the cloud, of course, but you're not really 'running your own' if you do that. GitHub take care of the server question (apparently they use a physical-server provider called Carpathia [0]), and because git always needs to be available but is only used rarely, the amount they charge you is probably less than the cost of running a dedicated server for the purpose.
And all that is assuming that a self-hosted GitLab is just as good as GitHub from the developer's point of view. It may or may not really matter, but GitHub is probably the more polished and feature-rich service.
Building a competitor to GitHub is possible, but not trivial, see SourceHut. (We've been talking about GitHub, but of course they're not the only Git provider.)
I can see only a few situations where it makes good sense to run your own Git/GitLab:
1. Your Internet connection is slow and/or unreliable
2. There are extraordinary safety/security concerns associated with your source-code (military avionics code, say) so you want to run Git in an isolated network (no Internet connectivity at all)
3. Related to point 2: You don't want your organisation's data to reside in the USA. (To my knowledge GitHub don't offer any choice about this, but I could be mistaken.)
For the average developer though, I don't see much upside. Having more control isn't a compelling advantage, it's another way of saying you have more obligations.
If all I need is a git repo, with some tools. Why pay someone to mess it up, when I can mess it up for free?
The moment you place it in someone else's work queue you are tied to them... and they might not care about your projects deadlines. Just like github.