Looking back when the last change happened: there was a subversion integration with git, but people actually switched to git for proper decentralised version control rather than use it. Then switched to GitHub to recentralise but that’s off topic.
I remember git-svn being pretty commonly used back in the day (circa 2007). At that point a lot of open-source projects were still using svn and if you wanted to use git locally, git-svn or something similar was how you did it.
My first experience with git was using git-svn to work with my company's internal svn repository, which I did for a couple years before the company stopped using svn. There was no internal desire for decentralized version control (rather the opposite, in fact; they wanted centralized permission management and such).
Not everything that uses GitHub/BitBucket/GitLab etc. is (re)centralized. You can have an internal company hosting service and a public hosting solution, for example. I've also used the decentralized capabilities to synchronize between two computers.
There are many reasons why GitHub (or something like it) are popular, such as:
1) not having to host the infrastructure yourself (incl. hosting it on AWS/Azure/etc.)
2) discoverability -- being able to follow people/organizations creating projects you are interested in; being able to search for projects ~ having these on various websites makes it harder to discover them
3) additional functionality/capabilities like static web page hosting (great for things like personal projects), and CI/CD workflows
