There is no vibration motor because it would not be intense enough to feel through the entire iPad. It would be too focused on one side of the device or barely felt at all. Vibration motors are not very big and consume battery, but the amount/size of the ones to make it work with the iPad would just be a big waste and not really add anything.
I have never in my entire life of gaming (and I started at about 3-4 years old) I have never cared about vibration feedback for a game. I think maybe a fishing game can benefit from it, but that's about the only instance where the lack of it would be a big deal.