If that's your question, the answer is that there's absolutely nothing in the way technologically: academics could easily form shoestring "journals" around a github README files with links to arXiv.

As others have said, the real issue is historical: for better or worse the currency of science is still peer-reviewed publications in prestigious journals, and a lot of those journals are still owned by Elsevier.

Obviously this is starting to change, but old traditions die hard.

[0] https://joss.theoj.org/