Y do they call it a hair cut when They cut more then one hair
Because a hairs cut sounds downright goofy!
Imagine saying " I got a hairs cut " versus " I got a hair cut." Hairs cut sounds stupid.besides no one would take you seriously if you said " I got my hairs cut."
The reason why they call it hair cut is because hair does not have plural. For example, you cannot say my hairs have grown, rather you will say my hair has grown.