iPhone / iPad utilized as bluetooth trackpad

It occured to me a few months ago, that I hadn't ever seen an app that makes it possible to use your iOS device as a multitouch bluetooth trackpad. I wasn't surprised that I could not find one in the App Store, but I am confused why the jailbreaking community has not yet done this. In my opinion, it would be awesome not having to run a helper application, also, lag would be minimal. I don't think there is anything making it impossible, I am not a developer, though. What do you think? Is there something I'm missing?