Because everyones average is different.
Most GPU reviews do give their power consumption at various levels. Its up to the end user to figure out what that exactly means for them.
Haha, what would 'average length of time usage" be exactly? Where do you get that number? A game that's easy to run doesn't put as much load on a system as a game that has the GPU and CPU usage at 100% all the time, how would you account for those differences?