Why do men harass women?

By getting more than just flirtatious and by ignoring indications that their advances are not welcome.
By unwilling sexual activities. That would include unwelcome sexual advances and requiring sexual favors from employees as a condition of employment or advancement. Similar applies at college if a female professor were to require sex from one or more male students as a condition for getting good grades. If a woman is a supervisor and the man gets fired or cannot advance unless he has sex with her, or she makes up lies on him accusing him of harassment for refusing to have sex with him, that is also sexual harassment. In highly competitive work environments, some employees file bogus sexual harassment complaints to try to get promoted. That is very nasty to do.