It is simple because a simple explanation is possible. Simple doesn't mean "intuitive to proverbial grandmother".

Git is clever, moderately novel and therefore unfamiliar (depending on your background), and simple. There are not many concepts present, and the concepts that are there are not difficult to understand, but those concepts need brief introduction because they are concepts that many will be unfamiliar with.

If you buy a checkers board it will come with a (very simple) rulebook. You aren't born with some sort of natural checkers ability, you have to learn it. Nobody would claim that checkers isn't simple though.