There are many reasons why. Here are some in brief:
1) The attributes in D&D and most other Method I games are not precise enough conceptually to be able to accurately model sex differences. Dexterity is a good example of this, since it mashes together a bunch of different abilities including gross motor coordination, fine motor coordination, proprioception and kinesthesia, all of which have different statistical ranges of ability between the sexes. Which one of these concepts should dominate?
2) The attributes in D&D and most other Method I games are not precise enough numerically to be able to accurately model most sex differences. Most sex differences are fairly statistically minor. To actually model the difference, there needs to be at least a 5.5% difference in maximum values (which is about the value of 1 pt. of a stat in D&D). Most sex differences are nowhere near there. There are a small number that are great enough to overcome this threshold, but these do not uniformly favour men in real life, whereas in old school D&D they uniformly do.
3) Sex-based differences tend to ignore obvious areas in which women do better than men. The most obvious are in life expectancy, overall health, and pain tolerance. If one wants to model sex-based differences in D&D using the "stat limit" system, then by the same logic by which women's Strength is limited, men's Constitution should be limited. I am, so far as I know, the only person who has ever proposed this, and I find it has no traction even amongst people who bang on about the virtues of sex-based attribute differences for realism's sake (who are all themselves men). Similar cases can be made for other attributes as well - Wisdom, where men might experience lower caps due to a greater predilection for serious mental illness.
4) Sex-based differences tend to assume that men are the norm and women are exception or alterations to that norm, when of course the opposite is true both statistically and embryologically. It would make more sense to treat women as the norm if we are interested in realism, and modify men's stats, rather than vice versa. 3-18 should represent the range of a women's capabilities, with men mostly overlapping with that range.
I will set aside the "It's sexist" and "It expresses hostility to female players" and "It tells female players that you think they are inferior to men" because they are all so obviously true as to not require further elaboration.
I don't see what sex-based attributes actually contribute positively to the game, since the "realism" argument here is so baseless for the reasons given above that one might as well right "Chicks Blow!" on your forehead in sharpie. It adds an additional level of complication during character creation, it is poor simulation, it causes one to avoid choosing to play a group comprising over half the species, and the differences between individuals is already captured in the individual's instantiation of the 3-18 range already. It's positive contributions to complexity of choice and gameplay are... Nothing.