How much downtime could it get? They might redesign the homepage a few times and break it accidentally, then it's done. They're not gonna redesign it again and again every week forever.
You under-appreciate how enterprise software development tends to go.. there's always new "features" to be had, KPIs to measure and no time to tackle tech debt...
Hosting your own does not guarantee you 100% uptime. It just means someone less expert than the GitHub folks will be responsible for bringing things back online. It also costs you time and effort, which is a problem if you're on a deadline.
Same goes for the question of where to host it physically. It seems unlikely your physical server will have better uptime than a virtual server in the cloud.
> 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.
I set up a self hosted Gitlab where on-prem was a requirement and it's surprisingly easy. Obviously another service that needs to be maintained and updated but as code repositories and connected services are a crucial part of infrastructure it makes sense to have control over that.
Not the biggest fan of Gitlabs UI but I got used to it quickly.
I find it a bit surprising that it is still a headline, sure it is a huge service used by many. Though as you write its not the first time this year. But apparently still people use a single service as their 'origin'.
Seriously, just consider self-hosting [1] rather than 'centralizing everything' [2]
[0] https://news.ycombinator.com/item?id=23604944
[1] https://news.ycombinator.com/item?id=23572532
[2] https://news.ycombinator.com/item?id=22867803