One of four reasons. 1) they're an incel. Women won't give them the time of day so they form resentment. 2) they were at some point completely crushed by a girl/woman emotionally and rather than deal with it and admit they even have emotion they make it her fault (justly or unjustly) and form a misogynistic view that all women are the same and they're projecting their hurt and hatred on parties completely uninvolved. 3) they come up short. In one or more ways and are compensating.4) to seem macho in front of their friends (even "grown" men will do this)