Nearing half the market of games that have protagonists? I think it's great that more women are gaming. And I'd love to be wrong about this, but it does seem like they mostly make up the casual end of the market.
I'd argue that the lack of strong, unsexualized and realistic female protagonists is the barrier here to begin with. The biggest argument I've heard from other women on why they don't play more "hardcore" games is specifically that they feel like they aren't represented in the scenarios these games present and when there are females, they don't look (or act) like they should if they were really in those situations. You don't tend to get a history of how she got there and what she went through. She's just there, in a tight little number, ready to jump around and look pretty.
The second complaint is not wanting to deal with the gender targeting that comes with gaming, both online and off. Girls feel like they can't be themselves (or even speak) in-game without being seen as or treated like a lesser competitor and they feel like they can't talk about them in real life situations without being labeled as a try-hard or someone that only got into it because of a boyfriend/brother, not because they actually enjoy it. You'll hear a lot of girls recount situations in which they're asked what their favorite games are, and when they answer with titles that aren't obscure enough, are told they're not real gamers. This is not to say that don't exist, but when you're treated like that's the norm, it's unsettling and makes you hesitant to bring the topic up in the future.
In having organized LANs for several years, I have dealt first-hand with the stares and questions dealing with my legitimacy in being there. Online, I've had entire teams target my player specifically and repeatedly with verbal assaults, kills and subsequent teabags despite having a gender-neutral name but a female avatar. It is not surprising at all to me why women would steer clear from this environment.
Third, a lot of women just don't talk about it because they don't see it differently from any other hobby they might have.
I think marketers just don't look hard enough to find who is really playing because they don't care. These tried-and-true formulaic games will keep bringing in the revenue they need to keep churning them out, why bother throwing a wrench in with inclusion?
It does suck that mainstream games nowadays are very macho-male oriented. But if you can look at the whole of gaming, there really are plenty of female protagonists, respectful and strong ones too. It's an annoyance of mine that games like Gears of War and Halo drive what most people think videogames are.
Online gaming is very male dominated. And yes places like Xbox Live are not exactly the best place for girls, unfortunately. But if you instead jump on Steam and play a game of Team Fortress 2 for example, it's a completely different story. Sexist, racist mouthy gamers are not always the norm. Different communities attract different types of players.
My fiancee loves the Tomb Raider movies. When I showed her that they are based on a video game series, she had no desire to play the games at all. I was careful to show her Tomb Raiders were Lara's boobs aren't ridiculous too. She has the opportunity to interactively play through the movies she loves, but that just doesn't interest her. Anecdotal, yes, but in my experience a lot of girls feel this way.
We're nearing 50% of the market, just so you know.