Why do girls hair grow longer than boys?

It doesn't. Girls just prefer longer hairstyles than boys.

Not necessarily true:

Girls do prefer longer hairstyles because it matches the woman style BUT- it also is a sign of womanhood/being femine. For men/boys, they have shorter hair because in the older days, they needed shorter hairs for activities they did ex: hunting, battling. But, many girls have short hair and many boys have long hair. We're equal--hairwise.