You're looking at that statistic as 18 percent of players want female protagonists, so 18 percent of games should feature female protagonists. From a business standpoint, that's simply wrong.
Hypothetically, let's say that 18 percent of gamers would rather play as a female lead character, and 82 percent of gamers would rather play as a male lead character. We'll forget about the actual percentage of female gamers for a moment, which I would guess would have be somewhere under 18 percent, if the ME2 stats are to be believed. Note that I'm only including "traditional" 60 dollar retail games here.
Now, knowing this about your potential audience, why would you go with a choice that is preferred by 18 percent of potential customers, over a choice that is preferred by 82 percent of potential customers? This is even more true when making a female character too sexualized could lead to accusations of sexism (Tomb Raider, Bayonetta), and not sexualizing a female protagonist enough could cost your game sales (Mirror's Edge, Beyond Good and Evil). Not to say that THE reason those games didn't sell more is because of their non-sexualized female protagonists, but come to think of it, I am struggling to come up with a game that featured both a non-sexualized female protagonist and was a fairly large success. Anyone happen to know of some?
Believe me, I'm not saying it's right, I'm saying it's a sad truth. Don't get me wrong, I love when games allow a choice between genders, and I would also personally like to see more female protagonists in games. With that said, I think it's a bit telling that I've recommended Mass Effect to a few male friends now, and despite my recommendation of going with fem Shepard, they've all played as men.
So not enough men want to play as women, and not enough women want to play video games. To borrow and reverse an old urban phrase, don't hate the game, hate the player.
But you have to consider -how many people buy games solely based on what gender the main protagonist is? Probably not a lot right?
Also while we're talking about "sexualization". I think that entire concept is just an invalid one invented by media. What defines "Sexualized"? A certain cup size? A certain degree of clothing? Where do we draw the lines? Why should we have to draw lines? I think I have yet to hear a single woman getting genuinely offended because a female character in some game showed to much skin. The characters actions is a different story, but let's be honest, in how many games outside of Duke Nukem and Leisure suit Larry do female characters actually act like sluts?