I currently work in developmental flight test. Any time we add a new piece of electronics to an aircraft (or replace an existing piece with a new version), be it a radio, a computer, or test instrumentation, we have to conduct an Electromagnetic Compatibility (EMC) Safety of Flight Test (SOFT) to make sure that the new equipment does not interfere in any way with avionics that are critical to safety of flight. Even if it's something that's not being installed per se (e.g. a laptop for collecting test data), it has to be tested. Sometimes there are exceptions, such as very minor upgrades, especially minor software updates.
Generally everything is designed so as to not interfere, but every once in a while something causes unexpected interference. Sometimes it's because the designers screwed up, sometimes it's because a bespoke item wasn't manufactured to the correct spec, and sometimes it's because of something nobody anticipated.
The problem with consumer electronics is that they haven't gone through this process. None of them should cause interference, but we can't be sure because they were never subjected to comprehensive EMC SOFT. Nor would it really make sense to spend that kind of time and money when we can just mitigate the risk of interference by having everyone turn everything off during critical phases of flight.