You are right that the default UI of git is intimidating for normal people.
Saying 'relational database' says about as much about how you actually store the data as saying 'json' or 'xml'. Yes, you could use a variant of git that stores all its information in a relation database. (And in a saner parallel universe, git might have used something like sqlite internally, instead of hand-rolling its own formats from scratch.)
But the question of UI is pretty much independent from the question of how you want to store the data.
You are right that the default UI of git is intimidating for normal people.
Saying 'relational database' says about as much about how you actually store the data as saying 'json' or 'xml'. Yes, you could use a variant of git that stores all its information in a relation database. (And in a saner parallel universe, git might have used something like sqlite internally, instead of hand-rolling its own formats from scratch.)
But the question of UI is pretty much independent from the question of how you want to store the data.