Having done some EMC shielding validation/verification on a component, what was apparent is that shielding might be satisfactory when initially tested, but a few years down the line when the shield (in this case a coating of silver-loaded epoxy) or seal has suffered a bit of wear and tear, it may not work anything like as well. This is particularly so if the people removing and refitting the shields and seals do not appreciate how they work and the properties of the RF it is trying to keep in or out. Just because a type of aircraft is considered EMC compliant, doesn't mean that particular airframe is. I should think the costs of ensuring continued EMC compliance for all systems on an aircraft would be prohibitive.
A few years ago, when the Nintendo Gameboy DS became popular, I went with the family on a package holiday. When we got there, Mrs Mechta's granddaughter seemed to know a lot more about the other children at the hotel than I thought possible. When I quizzed her, it turned out she had been 'chatting' to the others via the DS's wireless network facility throughout the flight, so its not just phones and laptops that are the problem.
A lot of the problem with things like phones is that their owners think they are switched off when they are not. My own will burst into life with some long forgotten alarm from switched off state, so when I really need it off (as in an exam recently) I take the battery out. Even then there may still be a further battery inside to keep settings.