At the very high risk of sounding like an MRA...   Can someone comment and/or share why child rearing can only be done by women?

Context. I have someone I manage in the US at work. He has court stuff going on as his wife is a alcoholic and not looking after their two girl toddlers. He's tried several avenues including couples counselling etc. to no avail. He is going to fight for full custody.

My personal experience is similar. My father won custody because my mother was not up to looking after two kids under three (which she, today, openly states). 

My father raised us (my brother and I). He did an OK job and he wasn't perfect. Neither are mothers. But, I would choose what I had over what my mother could provide.

Why are court systems designed to favour mothers? What advice can I give him for the US?