At long last I have an answer to a question that for ages has gnawed at my soul like a carrion-obsessed vulture.
It’s not personal grooming or self-massage that keeps people in public restrooms for such extended periods of time. It’s deep, probing philosophical thought, produced in the most appropriate of places. And then the expression of those thoughts, flowing freely onto the door via an indelible marker. For example:

 
What a perfect venue to share and air a bunch of shit.