You Don’t Need to Use Airplane Mode on Airplanes | Airplane mode hasn't been necessary for nearly 20 years, but the myth persists.

Here is a video from TED-Ed that explains why. TLDR: phones looking for a signal broadcast on high power, but planes move so fast that you can end up right next to a cell tower so the high power signal can interfere with other phones trying to communicate.

https://youtu.be/iKYHf22qVdM

I disagree with how he phrases a few things, but I think it's mostly accurate.

I thought another reason was to avoid blasting everyone* you pass with your phone's maximum power signal while trying to reconnect.

Edit: the problem being interference, not any health effects. I read that the effectively one-way signals from the sky worked somewhat like a jammer.