Planes use special radars called altimeters pointed at the ground to determine how far the plane is from the ground.

The FAA, which makes up all the rules for airline flights in the USA, isn’t 100% sure that the cell phone frequencies we use to make calls don’t sort-of overlap (and therefore interfere) with the altimeter.

Technically, the FCC (who regulate radio frequencies among other things) has made efforts to ensure that these “lanes” of frequencies stay separate, but manufacturers of altimeters were lax with their standards and the FCC was lazy in enforcing that altimeters in airplanes stayed in their lane, and that’s why you have to put your cell phone in mobile mode.