RF testing requirements and efforts vary from country to country, and even with certified devices there might be manufacturing defects that could cause interfer

RF testing requirements and efforts vary from country to country, and even with certified devices there might be manufacturing defects that could cause interference. So it's impossible to be 100% certain. The rules were there as a "better safe than sorry" measure, implemented at a time when most people didn't carry many electronic devices (way before cell phones became popular). The pilots have training to fly with multiple instrument failures, but even so you want to do what you can to reduce the risk of that happening. RF interference can occur in a wide area of the spectrum, so it's not really possible to eliminate it just by changing frequencies. Even if you could, getting the aviation community to change their frequencies which are used all over the world in all airplanes and at all airports would be exceptionally expensive.