Physiology. Same reason that while the men run with the women, the women are scored as a gender class in a marathon. Men on average are physically larger and have more muscle mass per unit of body weight. Nothing to do with anything but science. Some women can outperform some men ... but when you get to the absolute top of each gender, Most men are larger and stronger than most women.
The women can work just as hard, train just as hard, be just as competitive ... but there is that 10-20 percent physiological advantage men got through whatever source your belief system favors ... we can go with genetics, nature, breeding, inherited characteristics based on patterns of behavior, or the will of the creator or whatever. It is a measurable and undeniable difference, and in sports where it matters, it matters.
There is no reason a specific woman couldn't join the men's tour that I know of ... as others have said, if she can ride well enough to earn the spot, there it is.
I'd actually be more pleased if women's cycling got a lot more attention and coverage. That way there could be twice as many races to watch. Coverage of women's racing is pretty spotty. I don't know why advertisers think people won't watch a bike race because women are riding.
As far as I can tell, in Mountain Biking, the men and women get equal billing and equal coverage ... though the women do generally race fewer laps on the mountain bikes. Same downhill courses, as far as I know.
One of the top five MTB riders on the women's circuit is 45 years old, too ... destroying stereotypes with ever race finish.