First off, I'd like to promise that this article really shouldn't be as controversial as the title might imply. I won't be going into a discussion on the aesthetics of female miniatures (at least not here), so fear not and read on!
I'm sure most people who have been in the hobby have seen comments regarding the lack of female figures in some wargames, and I think there is a factor involved that some players may have overlooked. I know this argument has been leveled at Warhammer 40K for years, and since I'm reasonably familiar with that setting I'll be using it as an example. The general complaint is that many armies consist of entirely (or almost-entirely) male figures, and that females are underrepresented.
In my opinion, there are two main factors to consider with regards to the issue. First is a matter of lore: does it make sense for females to be found in the given army, considering the lore of the setting and that particular faction? Looking at Warhammer 40K, let's use the Space Marines and Imperial Guard as examples.
Space Marines are genetically-modified "human-plus" monastic holy warriors, who use gear passed down by their predecessors. Given those traits, I would argue that it does not make sense for there to be female Space Marines. And even if some Space Marines are females, you'd never be able to tell underneath their extremely bulky power armor.
On the other hand, the Imperial Guard consists of conscripts from the thousands of worlds of the Imperium. In a lot of cases, it's literally an "all hands" effort to defend Imperial worlds. Based on that, I would expect the Imperial Guard to incorporate any able-bodied person, male or female (and sometimes even children, though I suspect Games Workshop probably doesn't want to go there with their lore). If you can hold a lasgun, you're a valid conscript. So I would totally expect to see female Imperial Guardsmen right alongside the men.
So, what's the issue then? Where are the female figures? Let's take a look at that other factor: production. Specifically, the cost/effort/returns on producing the miniatures for the game.
Miniatures are typically made out of either metal or plastic (there is also resin, but it seems much less common). While metal miniatures tend to be the most expensive to purchase, they're the cheapest to cast in terms of mold cost. On the other hand, plastic miniatures tend to be much cheaper per figure, but require much higher up-front costs for the plastic-injection molds.
So why does that matter? Simply put, it's expensive to create new plastic molds. And most basic troop types are produced in plastic, because you'll need a lot of them and it's extremely expensive to play army-based wargames with only metal figures. Games Workshop generally has multi-part plastic kits for their basic miniatures, and these figures can be assembled in a variety of poses. This is great, but also means that every part needs to work with [almost] every other part in the kit. I say "almost", because there are usually a few combinations that just won't work. But more or less, every torso works with every pair of legs, every arm matches every torso, and so on.
Now imagine you have a mix of male and female torsos, male and female legs, and so on. Sure, you could try to skimp on costs and just make female torsos, but they probably won't match up right to male arms and legs (the latter will look oversized by comparison). So then they'd have to include slightly slimmer female arms, legs, etc. in the kit to match the female torso... which means there are fewer total possible combinations, as all of those parts couldn't be mixed with the male components. In an attempt to add more variety... you wind up sacrificing a lot more variety. So that's not a very good option.
An alternative would be to produce an "all-female" kit. And in fact, Wargames Factory does this with their Zombie Horde and Zombie Vixens kits, which I think is the best way to go about it. However, this means the high up-front costs have to be paid again for another plastic-injection mold (or two, or three... however many sprues the kit requires). That puts a lot of pressure on the second (female) kit to perform at least as well as the first (male) kit to justify the investment. In some cases (such as zombies) this is probably justified, but I can easily see where it might be a less certain choice.
On the other hand, providing some metal options for female troops can provide some variety for players who want it. And it might also be the best cost-effective way to determine potential customer interest in a plastic kit for a given army. But that means the female troops would cost significantly more than the plastic (male) kits, and players who are focused on value-per-dollar over variety are likely to stick with the plastic kits.
In closing, I hope I've given some sound thoughts on the subject and perhaps provided a plausible reason on why there aren't more plastic kits with both genders. I do think that "mixed" groups fit much more with modern or sci-fi settings, but tend to look out of place in fantasy/historical/historically-inspired settings. A lot of people have a very bad habit of forcing modern viewpoints on historical or fantasy settings, and most don't realize how terribly anachronistic and nonsensical this is. But that's an article for another day!