Initially people assumed without evidence that the wireless signals from portable devices would interfere with avionics. It was the days where you could prophetically "hear" that you are about to get a call or sms through your computer speakers.
If you don't have evidence that emissions will be within acceptable limits and will not interfere with the planes avionics then you don't introduce things into life safety critical areas like planes.
It's the same reason a captain can declare pretty much whatever they want on their plane and you as a passenger are _obligated_ to follow those orders. It's a felony if you don't. The captain doesn't have to present evidence just any concern that some action might interfere with the safety of his flight and that's the end of the discussion.
[1] https://www.quora.com/How-does-LTE-perform-on-high-speed-rai...
[2] https://www.huawei.com/en/media-center/multimedia/videos/202...
When I was getting my private pilot's license in '07, my instructor put his cell phone in the Cessna's glovebox (which is just below the radio) and told me to call him. There was definitely interference throughout me ringing him, but as soon as he took the phone out to cancel the call, the interference went away.
That's like configuring a firewall with a default-allow rule.
No, with any safety critical system you must assume there might be a problem until there is overwhelming evidence that there won't be.