I actually had a questions about this myself. (I wasn't so nice about it). Graphics for Laptops are built onto the motherboard. You buy the laptop with the graphics built in to it. This is fine if you know what you are paying for, and why you want those graphics in the first place! But Graphics take up more power, then up adding to the cost of the laptop (because you are paying for top of the line). Most people, and most laptop companies don't add all this unless you ask for it. The laptop companies want to sell you the cheapest laptop for a good price. So for the Average user that buys a laptop, the THINK because it is "New" it should be able to Run things fine! Unless you pay for the extra power of the graphics, you aren't going to be able to play those extra games. Even more, in the future if your newer games require better hardware the laptop is stuck in the past.
A desktop is more suited for gaming. You have your mouse, keyboard all laid out for gaming (I don't know how people play games on a laptop with out a mouse?) Plus if something is behind the times, you only need to upgrade that ONE part of the computer. It just doesn't make sense to me to buy some $2000 gaming laptop to play games for maybe a year, and then its behind. You buy a $2000 desktop computer, then just upgrade it over the years.