That's not actually true: cell phone rx/tx power is quite low. We can get away with that because all they need to do is get to the nearest tower, which has a ton of power, sensitive antennas, and is very tall. Amateur radios have far more power available to them, but any "p2p" (i.e. simplex in amateur radio) runs into normal RF issues, like obstacles and interference. If you used the existing radios in cell phones to communicate directly with other cell phones, you wouldn't get very far. Even amateur radios, with all their power, use repeaters to the same effect as cell towers.