Because "sex pest" is easier to say and more accurate than saying "Male feminist soyboy who rants on Twitter and gives shitty talks about how everything that makes a woman uncomfortable is rape, and complains about fictional harassment of women, but he himself constantly pesters women for sexual things that rarely cross the line into rape but are still shitty behavior including, but not limited to, asking for nude photos, sending unsolicited nude photos, asking if they can watch them have sex, asking them to perform whatever fucked up fetish he has, and trying to cop a feel when they're drunk. All of this while constantly accusing men of 'harassment' and 'misogyny' for behavior that is much more mild."