Because it is insulting to be approached by a man who is not in your league.

If a woman has a Ph.D in physics, she would be insulted by an illiterate cab driver approaching her. If a woman is refined and expects introductions before she is approached by someone, she will be insulted by a pick up line.

Many women will tell you it is insulting to be approached by a man whom she has no interest in.

The point is not why they are angry. The point is that you are bitter because they aren't interested in you.