Being a parent is fundementally different from friendship.
As a mother I am responsible for my children, my job is to protect them, nurture them, discipline them, teach them, make sacrifices for them, give my life and anything in the world for them whether I feel like I particuarly like them or enjoy their company or get much from the relationship myself at any given time.
This is so far removed from anything I would expct from a friendship that I really couldn't regard my sons as my friends.
The role of a mother is so fundemenatlly different, it's deeper, more complex, the love so intense and the desire to protect so deep, that I could not regard it as 'friendship'.
I have many friends, good strong friendships, but I'd never had any experience like being a mother until I became a mother. Being a mother is so different to being a friend.
Realtionships are not defined by what you do together, you can go to the cinema with your friend or your child but the underlying realtionship is still totally different.