That's partly a tooling issue, though. If git had native requests and a decent UI around them baked in, and the UI client also had some way of discovering your peers across networks, then the need for gitlab/github would be diminished.

Yeah no, unless you work on a project with 100 other people. Even if peer discovery, nat traversal and whatnot would be solved, what am I supposed to do if both my project members are currently offline? Synchronizing progress would be a nightmare in a three people project where everybody is located somewhere else. You could pretty much consider git peer to peer already, but everybody is too lazy to open their firewall and instead talks to the always-on supernode that is github.

Diminished. It's extremely common for teams to be online throughout the same business day. If you have an entirely async team where you can't coordinate time to exchange work product, then sure, you need an async third party location. That's by far the exception rather than the rule though.