Something to take into account here is that not all of GitHub repositories are actual projects. The huge increase in the past few years in the number of repositories in GitHub is not just related to a proliferation of open source projects, but also to the fact that GitHub has become the default tool for many people to simply store their work.
Nowadays, if I start a new online course, want to play around with a new language or framework, or just want to code something for fun, I create a new GitHub repository (public, of course, because why private?) to store it, and maybe share it with a few colleagues. There are even people who create a repository just to use GitHub issues as a ToDo list (though I find this to be a little too much). These are not projects or contributions meant to be shared with the world, and they definitely do not need a license.
What I mean is that before analyzing the number of GitHub repositories that do have licenses, we’d need to know how many of those actually need a license.