T-Shirt's with skulls on.

I go shopping with my boyfriend and i've noticed that there are lot's of men's T-Shirt's with skull' on them.
And i do mean a lot. Almost every men's T-Shirt's i pick up/ look at has a skull on it.
It got me thinking. Do men like skull's? Was the need of skull T-Shirt's too great to ignore? im not sure why there are lots of T-shirt's with skull's on them but they're there.