In a way, this seems like a "duh" article. But it clearly isn't, since we get gory game after gory game, and not much in between.
For example "Portal" was a joy to play. My first run through that game was like a continuous christmas morning, each level something new, challenging, and rewarding. The plot was revealing in tantalizing glimpses, in a way that pulled me into the game, wanting more. It was, quite simply, an amazing piece of videogame art. This is in contrast to most shooters, where the plot and the gameplay are just tools to get to more and more mayhem, and personally, I would take the writing and style of a game like Portal over a million blood splattered enemies.
But we get more and more FPS games where the differences really seem to mainly be superficial, a different engine, better graphics, but in the end are still "point gun and pull trigger" games. I suspect there are a number of reasons for this.
First, blood and gore sell to the target audience. Let's face it, your average gamer isn't concerned with nuances of plot and subtlety. They want to live a violent fantasy. So games sell them that fantasy, with flashy graphics and physics, but writing that could have been done by any hack.
Two, writing on that caliber is HARD work. There is a reason most books, movies and TV shows are mediocre. There isn't an infinite supply of talented writers out there, and for those who are out there, it's not really rewarding work. When we talk about how good a movie is, we talk about the director and the actors, not the person who wrote those words in the first place. This is more so with videogames. Who knows who wrote that dialog, or the plot? So most of the time, dialog and plot are put in second (or third, or fourth) place after graphics and physics. And because, as we've established in point one, violence sells better than nonviolence, when it comes to games, so there is more money out there for developers to make the violent game. Take Crisis. Absolutely insipid writing, terrible plot, but amazing graphics. It's clear where the money was spent by the developer.
Third, the replayability of plot driven games is much less than violence driven games. Once you know the end to portal, it doesn't change. Once you have solved the puzzles of Braid, the challenge is gone. But splattering your enemies can be different every time.
Personally, I want good writing and less violence in my games. Occasionally, good writing and violence can overlap, like in Bioshock, but I would like to see less violent, plot driven games out there.