Because they want to see everyone as inherently equal. And being different implies that some people are actually "better" in some areas over others, which would imply that inequality is an inevitable outcome.
Well it'd be actually pretty hard to test any of this in a scientific manner due to both the need for double blind controlled studies in environments that are not conductive to such and some way to avoid all the feminist limitations on researchers (IQ test scores and gender are such examples. And no I don't really believe most guys here want anything less than equality by law, it's just frustrating to see one girl after another fall for the badboy/jerk and dump her more nerdy/caring boyfriend.
