It's my understanding that grouping by weight is done so that the sport emphasizes skill more than sheer muscle mass. I mean, no matter how skilled a featherweight or bantumweight fighter is, a heavyweight connecting a solid head blown is likely to end the fight with a knockout, while the reverse is unlikely to be true. That's not what the sport is supposed to be about.

As to grouping by gender, I don't know. I suppose it's the perception that a man with a mass of X is more powerful than a woman with a mass of X.

We know this isn't necessarily true for athletes, but such is the power of cultural inertia.