Because putting them together in most physical sports would push women out of the highest echelons of that sport. Just look up what female MMA fighters and female tennis players have to say. They literally can't keep up with men. Serena Williams and her sister boasted that they'd beat any man outside the top 200, Braasch (then #203) took the challenge and on the day of the challenge played a round of golf drank 2 low ABV beers before easily beating both sisters

Probably the most detrimental thing you can do for women in sports is to get rid of the women's league. Most "men's" categories are already open for women, so you should ask women why they don't want to partake. The answer is what female athletes already say, they'd get absolutely dumpstered before they even get close to the top. Of course the less physically demanding the closer men and women will be, but for most sports the physical differences make women's leagues necessary.