I never thought I'd think about them in the same thread.
Urine is waste material that the body is disposing of. BM is provided by the body to nurture a child. I wouldn't compare urine with cow's milk either, we don't consume urine. BM is made for consumption, urine is waste. I have a problem consuming waste.

ETA: I also wouldn't share my urine with someone or expect them to feed me or my child with urine. The mind boggles at the thought.