Throughout the history of video games, a large percentage of games (if not most of them) have targeted the male 18-34 demographic, meaning that it often behooves developers to include sexualized female characters. Some marketing uses these characters so prominently that it has little to do with the game being advertised. An example of this is Ninja Gaiden Sigma 2's advertising focusing on the motion-controlled breast jiggling feature, while the actual game is about being a ninja.
Once games made the leap to 3D, it became easier for games to render implausible female figures (a far cry from pixel art bikini Samus). Characters like Lara Croft began appearing, who dressed in tight shirts and shorts to emphasize her figure. One of the major selling points of the Dead or Alive series has been the gratuitous clothing worn by the female fighters.
The outfits that these characters wear are usually ridiculous and seem unfit for the task at hand; a much-ridiculed theme in fantasy games is the bikini-like armor worn into battle by many female characters.