The mailing list that they use now is centralized. It works by everyone sending a message to a single email address. The owner of that address then sends the email out to all of the subscrbers of the mailing list.
Having everyone who wants to upstream their patch directly to the kernel submit their work to a single website is not much different than them sending it to a single email.
This here for example is sent to Linus directly and that part does _not_ depend on the mailing list. The mailing list is just a convenience feature for people that _want_ to be aware but may be looked over/forgotten by the submitter (which still happens, and people regularly say stuff like "also CC'ing additional relevant people").
Yes you could, at least in principle, also do that via GitHub/GitLab/whatever. But a) you wouldn't gain anything (the kernel already has all the tooling it needs and it's large enough to just create missing tools, heck git was literally written for it), b) you would introduce a massive single point of failure that either i) is _not_ under control of the kernel.org team or ii) needs a massive amount of resources to host (see the fun people poke at displaying 2k commits in GitLab). Neither seems like a good use of the kernel development resources. And if you want to contribute to the kernel... using a mailing list is an absolutely trivial problem.
This sort of implies that the Kernel would use something centralized if it worked well, which I don't believe to be true. Using email is very much an intentional choice and a desirable solution.