I don't doubt that electronics may cause interference. Thing is, the airlines may not know when they may or may not interfere and to what extent.
Years ago back when I was in college I worked at a McDonalds. One time while working the drive thru, I started getting some interference and hearing parts of another conversation over the headset. Someone in the DT lane pulling up to the window was talking on their CB radio and that was coming over the headset. It was so bad I couldn't hear the person at the speaker placing their order and had to ask the person on the CB to turn it off because of the interference. That was the only time I remember anything like that happening.
These days there are a lot more electronics around, and I'm surprised there isn't more interference now. If interference is unpredictable and most emergencies happen on takeoff and landing, it makes perfect sense to prohibit the use of electronics at those times.