Well written GitHub issues and PRs that you can reference in your git commit also don't save you from commenting code. I mean please do not reference an issue in a code comments, but describing to other people what a function in general does goes a long way to build understanding.
For new contributors it's probably also more useful to see the latest snapshot of the code base and code comments than having to check a git log in order to find out why a function would be written in the way it is.
I'll absolutely reference an issue number in a comment (with some context comments as well) in cases where it makes sense.
Sometimes you need to go back and read through a lot of context understanding why a block of code does what it does, and it's a lot more efficient for the rest of the time to have a quick reference to the full explanation rather than pages of comments giving that context.
For new contributors it's probably also more useful to see the latest snapshot of the code base and code comments than having to check a git log in order to find out why a function would be written in the way it is.