Maybe they're hosting in us-east-1 though :)
Self-hosting a service like GitHub that operates at GitHub scale is difficult.
Self-hosting a service like GitHub that operates at the typical small/medium company's scale is trivial.
A single machine (with separate runners for CI) will cover many companies' needs. It being a single machine eliminates a lot of the complexity and failure modes associated with a distributed system and makes backups/restores/maintenance easy.