Why does the world expect you to have a mother AND a father?

I haven't lived with my father since I was 4. My brother was born the day after he left. He was a deadbeat. But now Im 16. I love my mother, she is my life. But i am getting fed up with having to earn the second income, juggle school and work. I have to do all of the things he never did. But nothing is made for single moms and their children. The world treats us like we are expected to have a father. Why is it like this?