There are at least five such front ends (easygit comes to mind) and extensions (like legit from Kenneth Reitz). But none is popular, and I suspect the reason is

There are at least five such front ends (easygit comes to mind) and extensions (like legit from Kenneth Reitz). But none is popular, and I suspect the reason is that, despite all the complaining, Git's default UI is not actually that bad in day to day use.