Yes. Bandwidth and latency. It's just that simple.

1) The earth is not flat, it curves, so when radio waves are emitted from a cell phone tower antenna in all directions, the majority goes into space and gets blocked by the ground. Only a little bit goes perpendicular to the curve of the earth at that point, but then the curve of the earth drops away from that straight moving signal. So a single cell tower is likely good for about 6 miles in any direction. The three miles is for good coverage for the handoff to the next tower if the phone is moving like in a car from one tower to the next and objects like buildings or mountains may block signal.

2) A strong power single tower attempting to cover all the connections of cell phone use at any given time of the country would not have enough radio spectrum bandwidth to contain all the information. Rather many smaller weaker towers more efficiently use the radio spectrum bandwidth.

The biggest reason is line of sight. Calling someone on the sphere of the earth requires more antennas because of both the curve of the earth and obstacles. The moon has very clear line of sight from the earth generally. But even the moon could be on the other side of the earth in which case we could only make the call from a relay antenna on the other side as well. So essentially the same thing.