Hair clog in the bathroom sink

Do you ever wonder why the bathroom sink drain clogs with hair? On the pages of my site, <Plumbing-geek.com>, I talk about the drains that are plumbed in iron pipe and how that kind of pipe tends to accumulate the hair that goes into it. The question is though, unless you are washing your hair in the bathroom sink, why does it have hair in the drain?
My 1935 house has a large main bathroom with a vintage cast-iron pedestal sink. The sink has it's attendant lit mirror but the bathroom also has a built-in vanity with it's own lit mirror. I brush my teeth at the pedestal but I brush my hair at the vanity. Because I don't brush my hair over the sink the sink does not receive hair and the drain does not clog with it.
Most bathrooms are smaller than mine but placing a second mirror anywhere else and using it to care for your hair is going to have a relieving affect on the amount of hair that goes down into that iron pipe, causing you to deal with that unsavory job.