Congratulations, you can configure your own mail server. You're already one in a million. Out of that group, maybe one in a hundred can do it nearly as securely as one of the big services. This is the reason why people use the big centralised services, and also the reason smaller ones don't survive: email is ridiculously over-complicated. You won't convince anyone it isn't by explaining that you managed to DIY in under a week.
Fortunately, rolling your own git isn't nearly as hard, and the percentage of people who could run a node compared to total users is larger. Hell, every person using git also has a fully-functional, secure and standard self-hosting stack already installed. Getting a GitHub-like web interface is similarly easy. This fundamentally reduces the risk of centralising, which conversely means that people don't have any qualms about using GitHub, since they can leave at any time. (Less true of issues, PRs, community, but hey.)
In either case, centralising has large benefits: one for handling complexity, one for giving you a community.
When you want decentralisation, it's because you don't want a single entity controlling what the ecosystem becomes. The circumstances where this is true are typically big utility-like things, where we care more about guaranteeing that water and electricity exist than the top end of providers' profits. Email is like this. You would never build your own power station, just like these days it would be bordering on negligent to use anything except Office365/GSuite/etc as your corporate email host. Lots of jurisdictions will create highly regulated marketplaces so you don't get Enron-California-style anti-competition. When things get that bad in Gmail-land, let us know.
[Edit: I was pretty snarky just there, but not at you! It was in general, I swear.]
Fortunately, rolling your own git isn't nearly as hard, and the percentage of people who could run a node compared to total users is larger. Hell, every person using git also has a fully-functional, secure and standard self-hosting stack already installed. Getting a GitHub-like web interface is similarly easy. This fundamentally reduces the risk of centralising, which conversely means that people don't have any qualms about using GitHub, since they can leave at any time. (Less true of issues, PRs, community, but hey.)
In either case, centralising has large benefits: one for handling complexity, one for giving you a community.
When you want decentralisation, it's because you don't want a single entity controlling what the ecosystem becomes. The circumstances where this is true are typically big utility-like things, where we care more about guaranteeing that water and electricity exist than the top end of providers' profits. Email is like this. You would never build your own power station, just like these days it would be bordering on negligent to use anything except Office365/GSuite/etc as your corporate email host. Lots of jurisdictions will create highly regulated marketplaces so you don't get Enron-California-style anti-competition. When things get that bad in Gmail-land, let us know.
[Edit: I was pretty snarky just there, but not at you! It was in general, I swear.]