That's not entirely true. First, increasing amounts of haptic feedback can be faked with vibration.
Second, the surface keyboard had raised keys. That's very useful for quick operation where you are not looking at the keyboard. Something that I don't think you can do with a tablet without sticking things on the screen.
A keyboard in turn has limitations over game controllers. Why not provide a game controller interface for your tablet instead so that manufacturers can provide arbitrary controllers instead of just flat ones the same size and shape as the tablet?
Faking haptic feedback with vibration sucks insofar as I've experienced it: laggy with unnatural sensation. I'm sure it could be done right, but a phone vibrator isn't it.
Second, the surface keyboard had raised keys. That's very useful for quick operation where you are not looking at the keyboard. Something that I don't think you can do with a tablet without sticking things on the screen.