That’s because a man who says “But I’m a nice guy!” is exactly the same as a fat girl who says “But I’m so smart!” It’s an attempt to emphasize a characteristic that has no real influence on the decision-maker.
There is a video clip of a 30ish old woman, with her biological clock ticking, around the interwebs saying that ladies are tired of the jerks after all these years and that they like nice guys, and “Thats you!”. Its totally insulting.