Only because of outdated, senseless, intrusive government regulations that probably don't even apply anymore.
When I worked for Jack Goeken(founded MCI & AirPhone) they told the story of "The Goeken Message". "The Message" was arranged to be delivered on all flights prior to takeoff warning passengers to turn off their cell phones. Most everyone has heard it, it goes something like this: We also ask that your seats and table trays are in the upright position for take-off. Please turn off all personal electronic devices, including laptops and cell phones. Smoking is prohibited for the duration of the ...
The "turn off your cell phones" part was part of the deal Jack made with the airlines to install his AirPhone system. Jack knew that cell phones would work from the airplane to any of the cell towers just below at ground level and the passengers with cell phones wouldn't use his high priced system. The entire thing spiraled into some insane concept that somehow personal electronic devices could interfere with with the airplane's sensitive instruments, as if the FCC would approve such a device for the mass market, GOOD GRIEF but rather than reveal the truth of the origins of "The Message" it worked out well for AirPhone so no one said anything about it.
The company is long gone now but "The Message" continues to plod on like an un-dead zombie, senselessly inconveniencing millions of air travelers for generations to come. One day in the distant future they will probably be giving "The Message" to space tourists on their way to visit the Moon or Mars. Like circa 1800s outdated fax technology it seems nothing can stop it now.
Bad actions can be (forgiven and) forgotten but apparently a bad idea can live forever.
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You certainly know more about this than I do... With the low transmitting power used in todays cell phones, there certainly would be almost no chance of interference with any aircraft systems, and that never was the issue even when using the high powered bag phones of yesteryear.
We used to always take our bag phones along with us in our private aircraft for backup emergency communications. It was amazing how when you were flying at 10,000 feet over the mountains or in the middle of nowhere... on those non-digital cell networks of yesteryear you could still get a signal and have a decent conversation. The caveat was that roaming back then typically cost $2 a minute or more. And anytime you were outside of a major urban area you were roaming.
Around urban areas the problem with using a high powered non-digital bag phone in an airplane in those days was that your phone was capable of connecting with every cell tower in a 20 to 40 mile radius. In those days this could cause problems for the networks which had a much smaller capacity. Before digital communications techniques made it possible for cell towers to handle thousands of users... a small amount of people could jam up the networks, especially if they were in airplanes which the protocols were not designed to handle. But that is the original reason cell phones were not permitted by carriers to be used in airplanes... network congestion. But as you say as soon as cell networks went digital it was no longer an issue. But the new phones also weren't powerful enough to make a call unless you were over an urban area.