This isn't really any different from any other market. In general, unregulated markets in the context of a property framework that allows for unrestricted accumulation of capital tend to be monopolized over time. And, of course, such monopolization will tend to correspond to centralization of services.

With your GitHub example, I think the better question isn't why John doesn't want to run their own server - that much is obvious. It is, rather, why doesn't John want to go to hosted GitLab instead?