Yeah, yeah and so are seat belts, warning signs, and dosage directions.
/facetious
It really is for the safety of the passengers. They're just not completely honest about the reasoning. I really doubt that the airlines want to start talking about takeoff and landing crashes right before getting airborne and how your phone might as well be a brick when it goes flying through the cabin and makes some poor chaps head look like a mellon at a Gallagher show.
Once upon a time the electrical signals probably did cause enough interference to warrant keeping them off during landing and takeoff when the plane needs to have good communication with the tower. Things have gotten a lot better and that's obviously not the case. It's unlikely that a phone is going to cause enough interference to cause a crash, but it's also not something I'd care to chance.
If you want to talk about completely unnecessary idiocy as far as security and safety at the airport goes, the TSA is much lower-hanging fruit.