git is decentralized by design and yet we ended up preferring centralized hosting
I think most people prefer tools that are convenient to use. Centralized code hosting services are super convenient. They offer many useful social features that make it tasking and unappealing to use git as intended. Even with tools like Gitea, we still flock to GitHub.
I think there is an administration cost savings as well. If we used git in a truly decentralized way, we would have to all announce and resolve each others remotes. The simple solution is to have a centralized service that we all connect to, al la GitHub.