Why turn mobile phones off on take off and landing?
It is unlikely that phones and other personal electronics could interfere in a way that would significantly impact communications, navigation or other systems. But, the possibility is still there, and that possibility forces the FAA to impose regulation. In most cases, device-use regulations are passed down to the airlines, and it is the airline that sets rules for device use on their flight. It's quite possible they limit device use during critical flight phases like takeoff and landing to help ensure people pay attention to safety briefings.