Maybe I'm paranoid, but it may also have something to do with overwhelming the customer with information. Imagine both ATI and nVIDIA sold three different chips each. It would be easy to get an overview of all the offerings. But by offering 12-15 different kind of chips the customer may think that they only have the time to investige one of them two.
I feel that way when it comes to laptops. Ok, I'm a Thinkpad junkie and loyal to IBM/Lenovo but it's still fun to look around still. Go visit Dell's website at hit the laptop section. First you have to choose whether you're a home customer, small company, large company or goverment. This choice will affect the offerings, i.e. you can't buy a Precision or a Latitude as a home customer or a XPS as a company. To be able to get a proper view of all the products you have to click around quite a lot. This takes time and is, in lack of better words, f**king lame.
Well, as a home user the options are XPS, Entertainment and Basic. XPS seems to be the gamers choice, lets pick that one. And over to XPS M1710. Hey, they have four different options. Let us see, this one offer a red top, and this one a dvd-burner. Which video card should I choose? Options, options.. Dell has the largest market share and they know that more or less everyone looking for a computer will visit their site. Using a number of different options they will try to break the user, preventing her from checking up other companys. This is taxing for the user, especially a user that doesn't know as much as most of us do. I find if quite hard to track the deal breakers when looking for a computer.
I say this as a challenge to you all, imaging you wish to buy a laptop with bluetooth, trackpoint, DVI and a weight less then 5 lbs. How long will it take to check Dell's site to see if they offer this?