My understanding is that Github called this "pull requests" because it does something similar to the `git-request-pull` function (See https://git-scm.com/docs/git-request-pull).

I quote

> Generate a request asking your upstream project to pull changes into their tree. The request, printed to the standard output, begins with the branch description, summarizes the changes and indicates from where they can be pulled.

GitLab, on the other hand, _does_ call it "merge requests" because you to your 12-year old's point, that is what they are.