cpu socket is different
the cpu socket attached with a large and heavy heatsinks at most weigh 1kg
the graphic card + cooler can weigh more than that.
plus, a cpu cooler can use bolt thru kits which have more clamping force at the base of the cooler.
graphics card lack that clamping force to keep it in place.
trust me, i know, i have an ATI 4870X2 with arctic cooler accelero xtreme 4870x2. the whole beast tilts down on the left side (the side hanging in the air) even when i'm using 2 screws.