Because not all hair follicles are affected by the same 'stuff' ... also, gravity helps when you reach a certain age, making it sprout from ears & nose too.
Male Pattern Baldness - the inherited type - is genetic and hereditary.
I believe that scientists are unsure why it only affects the top section of the male scalp, leaving the sides and back, and of course the face and other areas, to grow hair.
The fact that the face is unaffected allows bald men to grow beards.