bsmooth1, the reason is that devices with transmitters (like telephones) can potentially cause interference. So can a lot of other electronic devices; even circuitry that's not designed to be a transmitter can generate incidental radio signals. That's been proven in studies by organizations like NASA, which has recorded incidents where devices such as CD players have caused false navigational readings on equipment in the cockpit.

It may be a low probability but why not make it a zero probability by leaving all such equipment off in the most critical stages of flight, which would be at low altitude during takeoffs and landings? And since you can't use your phone at altitude anyway (as has been noted, the system on the ground isn't designed to handle your signals from up there), just leave the phone off from when the door closes until when they tell you it can be turned on.

"They just trust us the we will turn them off? " Why, yes, they do.

Are you saying you aren't trustworthy?