Because the iPad is meant to be held in the hand.
The main reason for the iSight cameras on Macs or Apple monitors is to do video iChats (yes, I know about Photo Booth, which people use once, go “Huh.” then forget about). If you were to try to do video chats on an iPad, held in your hand, the video would shake and move all the time. And that’s a bad user experience. Apple doesn’t do those. You can fix the problem some with image stabilization software, but probably not enough for a 10-minute chat using a camera forced to have a narrow field of view. And a stand would be a simple solution, but they’ve mostly come out of the gate positioning it as a device that allows you to do anything you want to do with it while you hold it in one or two hands.
I have no doubt that there are prototype iPads with cameras in Apple’s labs. But I’m guessing they haven’t licked the problem of shaky hands to their satisfaction yet. Give them a year or so.