Yes, this is true to an extent. But price is one of many factors that go into GitHub's design decisions for infrastructure planning. (I used to work on the Git Infrastructure team at GitHub, which is responsible for exactly this.) Nor is it the _first_ consideration. Price is important (obviously) but only after performance and resiliency.
Putting repositories on bare metal on-premises makes sense for GitHub's architecture. They run git itself, which is happiest packbuilding with as many IOPS as it can manage. But they've also built out a number of custom performance monitoring and load balancing applications, and any single Git repository is replicated across a number of servers using spokes, their custom Git load balancing system. They run on-premises because they've done a detailed analysis of their needs and hired a staff of developers and infrastructure experts to deliver it. And as a result, they're the world's largest Git hosting provider.
But you _can_ be perfectly happy in the cloud.
Visual Studio Team Services hosts its Git repositories in Azure. Since VSTS didn't start out hosting Git repositories, but Git repository management was added _after_ it already had a cloud-first architecture, it didn't make sense to host repositories on-disk. (Nor would relying on Git for Windows have been performant five years ago.) Instead, VSTS built out custom Git server implementation to take advantage of storing repositories in SQL Azure and Azure Blob Storage. And this implementation scales up to the needs of Microsoft's customers - both external and internal - who are hosting the world's largest Git repositories.
So I think that you can be happy on-premises _or_ in the cloud. I think it comes back 100% to your point that you need to hire for that expertise.