Lmao it’s because you’re too high. Signals move at the speed of light, the speed isn’t the issue.
Actually we are both correct. What I mean is that they are moving so fast that they go out of range from one tower to the next. A tower has antennas and how the antennas work is the networks switch (hand off signal) from one antenna tower to another as you move from one location to the next. The process is seemless cause you aren't moving fast. This is where the whole pinging comments comes from when they talk about crimes and such aka pinging towers.
Second an antenna is like a queue. Meaning as calls are being made folks signals wait in a queue and if so many signals are in a queue this is where there is an overload of signals and often times where you need more antennas. You see that at stadiums and other places where large amounts of people gathers.
Basically a plane is moving so fast the network can't switch from one antenna to another fast enough. Also cell tower antennas have a maximum transmitting range of like up to 45 miles. They also transmit horizontally and not vertically. So that is also as you said concerning height.