A whole generation of programmers have only ever known Git and GitHub. They assume that since it is the standard it must be good. This is a fallacy.
Bad things can become popular and become entrenched even when better things exist. Replacing Git today would require something not just a little better but radically better. Git was always worse than Mercurial. It won because of GitHub. If MercurialHub had been invented instead we’d all be using that and would be much happier. Alas.