Or: There are legit reasons to write your own library
var header = $('header'); and hit some kind of weird edge case?
I’ll be clear: I do not recommend writing your own library, framework or other massive code base. Not on a whim, at least. It’s not worth the time to do something that is only half as good as what already exists.
However, you cannot call yourself an expert at anything without knowing how the guts work. Knowing how fuel injection works does not make me a champion race car driver.