There are a number of reasons.
First of all is precision, despite what you said motion controls are MUCH less precise. You mention pointing at the screen which you claim would be more accurate. Would you mind telling my how you would move or turn your body in game? At most you might have one thumb stick to use (wii, PS move), which would let you move forward/backward and strafe left/right. Now if your arm movement aims the reticule on the screen, how the bloody hell are you going to turn? I played COD 3 on the wii and how they did this was if you aimed in the far corners of the screen it would turn you, which was a complete PAIN IN THE ASS. It was possible to play, just take 10 seconds every 10 seconds to orient yourself in the direction of a baddie. This greatly slows down the rate of interactivity and does the exact opposite of immersing me in the game - it pulls me right the hell out and frustrates me beyond words.
Second, motion controls are far form perfect at detecting what you want it to do then doing the appropriate thing. I can't count the times I've had to re perform a gesture to try and get a game to do something (in the rare occasions I played a Wii game).
Third, if I come home from a long day of work the LAST thing I want to do is to flail my arms like a retard in-front of my TV. Admittedly, for FPS games it wouldn't be as bad, I could sit with my arms in my lap (unless using KINECT) and move from there (depending on the game) but that is still more that I would like.
I believe Yahtzee put it great in his latest video (E3 2010): Game developers should be focusing on having us making smaller movements not larger ones, shortening the seperation between thought and on screen action. I think current controllers are fine and there's lots of room for innovation inside games for control schemes to improve handling.
However, I do think kinect is an amazing piece of technology... I just don't want it fucking up my gaming experience.