Why aren't there more stringent checks for switching off electronic devices during takeoff-landing?
There are some problems with this as electronic equipment are not banned in aircraft per se, but only during some (critical) phases of flight.


First, how to determine if someone is operating electronic instruments. It'll be difficult, considering that everyone is seated. The only way is to put a lot of cameras (who's going to monitor them?) or by asking everyone to check their fellow passengers, hardly a good option.
Even if someone operating an electronic equipment is detected, what to do? During takeoff and landing, seat belts are mandatory and the only option (except announcing the seat number over PA) is to ask the person in nearby seat to snatch the offending equipment; again not a good solution. 


As most of the electronic equipment (like camera or music players) emit little radiation and no air crash has been attributed to use of electronic devices, I doubt airlines will do something to physically to prevent passengers from using electronics.