First of all, its a lot easier for the graphical quality of games to improve than it is for AI to improve. You can only make something so intelligent before it will start hogging a lot of resources and it has to be pretty intelligent to out-perform human players that are familiar with the game. Humans are unpredictable and the AI will always struggle to anticipate what we are going to do. Developers usually compensate for this this limit by "cheating" and give the AI more abilities/less limits than the player, e.g. unlimited ammo, faster movement, more HP and greater numbers. That being said the basic AI should be a lot better with today's hardware. It all comes down to the available time and money at the developers disposal as well as the importance of the AI to the general gameplay.