Laptop graphics chips are usually based on mid-range or low-end rather than high-end desktop parts. For example, the top-of-the-line nVidia GeForce FX Go5700 is based on the mid-range nVidia GeForce FX 5700, and the top-of-the-line ATI Mobility Radeon 9700, despite the name, is based on the mid-range ATI Radeon 9600 XT. Granted these are old cards, but the principle is the same.
Laptop graphics chips are usually optimized to use less power through low-power components and power-management features. Even the best laptop graphics chips are not capable of running 3D applications (games) at the same speed as the most expensive desktop graphics cards.