Isn't the only reason it will go down be because of network issues or power failure? What other possible cause could be for system failure?
I have been running HA, Pi-Hole, Maria-DB and my own API instances on my Raspberry Pi and in the 22 days of uptime till now, there have been 0 failures
At that point you'd have to create and manage a cluster.
You have to update servers, etc. etc. and if you count the hours is many times more than working locally and wait a couple of hours until technicians at GitHub fix the problem for you.
Also, most people don't need to provide access to hundred of thousands of users so they won't ever need a cluster of Git servers.[1] (Some bloated UI like GitLab may need more computing to host even for a moderate amount of users, though).
Self-hosting Git is easy. Besides power failures there is not much reason this could go down if you don't help with that actively. But if you don't touch it besides OS updates it can run 24h a day for many years without any effort. (Biggest issue is backup actually, but you have to think about that point anyway if you run something on your own).