There are two significant areas - not what you think.
Interference with cell-phone towers
Cell phone technology puts a staggering number of radio transceivers sharing surprisingly little bandwidth. How does this work? By reducing transmit power, and having more and smaller cell towers. That's a big part of 3G/4G/5G. In the old days, an AMPS cell phone might have had 20 miles of signal radius. Now if your 4G phone is 1 mile from a tower, it down-tunes to 1 mile of radius. Your phone speaks softly, because it doesn't need to shout.
That way someone else 2 miles away can use that same bandwidth, which they couldn't under the older systems. However, if you're in a bad reception zone, your phone may punch it up high power with 10 miles of throw. That means no one else in a 10 mile radius can use that frequency: your phone is "stepping on their signal".
So there you are, at 7 miles of altitude. Your signal has to punch through seats, carpets, several layers of aluminum, a bunch of mixed cargo, and the fact that the cell antennas are aimed at the ground not the sky (so attenuated)... then make it 7-10 miles (albeit line-of-sight) to a cell tower. So your phone will punch it up to high power: it will be shouting.
That means you're reaching several towers at once, in nearly equal strength since it's line-of-sight and mostly "up", and sweeping maybe a 7 mile radius on the ground with that loud signal, so about 150 square miles. That frequency is unusable in that entire area, because you are using it, and stepping on any other signal that tries to be transmitted.
The tower system is designed to "hand-off" your cell phone signal to the next tower, as you travel... by land ... at sane speeds such as 70 MPH. On the ground, it figures out which tower to hand you off to based on your directionality and which other towers you get good signal from. But in the air, it's very unclear: you have good signals to towers all over the place - should your next tower be east, north or west? Further, this handoff is occurring much more often than the system is designed for, because of your 500 mph speed.
So a cell phone in the sky confounds and overloads the cellular networks.
Lithium battery fires
The closest real life gets to a horror movie is fire on an airplane. It's nightmare fuel for any pilot. There have been several incidents with massive loss of life on the runway, after a successful landing -- take Saudia 163, the plane screeches to a halt, firemen line up to help people down the slides, and the door ... never ... opens. There have been more incidents where crews detected smoke, but dawdled or favored a convenient airport, and that delay allowed fire to destroy the flight controls. UPS 6 (which could've made it to an Iranian island), Swissair 111, Air Canada 797, etc.
There is an aviation safety reporting system for crews to report genuine safety issues. It's not anonymous but reports are protected by law. NASA, who runs the system, has done aggregate analysis of risks relating to cell phones. From their report, the #1 risk is battery fire.
In fact, that's why there's a watt-hour limit on the size of accessory battery boosters you can bring onto an airplane.
They do have containment methods for burning consumer devices, but that is not a cure-all: Fire is a wildcard. You never know for sure what fire is going to do, when combined with all the random things consumers bring onto airplanes, and that consumers do. Imagine if a customer tries to self-manage the burning cell phone by throwing it into a galley trash can. Fire + panic = anything can happen.
Now, this seems hypocritical, since lots of consumer devices contain lithium batteries. But cell phones are the most prevalent, and the most likely to be operating unusually (cranking up the transmitter to max, as discussed).
And keep in mind, aviation has a long track record of underestimating the threat of fire. So over-reaction it may be, but the alternative has not worked out well for aviation.