Decentralized networks benefit from hubs if they benefit a subset of the network, which GitHub has for a long time. A hub is a focal point and there can (and should be) many of them in the git "network."

Shrug

Nothing prevents usage of GH in a decentralized fashion. There's nothing magical about git remotes. Just add some more, figure out a process that works for you, have fun!

In reality: when I want to send a letter I don't want to figure out a process from scratch. I want to go to the local post office, buy a stamp, and post a letter.

Convenience is a spectrum and different people land in different spots. What irks me is when I lack the choice. And that's not the case here.