You're crusing at 6+ miles in altitude typically. Cell Antennas can reach further than that, but they are directional. If a certain site doesn't have a downtilt, you can get a sniff of a signal at airline altitudes. Most cellular arrays have a down tilt of 1-6 degrees. The amount is determined by the network's needs and the surrounding area.
If an area is dead flat and very sparsely populated, they'll build tall towers and the antennas will have little to no down tilt. This maximizes coverage at the expense of capicity. The capicity isn't needed since the area is flat and sparesely populated, so you design for maximize the coverage footprint to reduce the number of pricey towers needed.