Because cell towers transmit waves omni-directional (not focused), and are also relatively low powered transmissions. When a dish is used focus the transmission waves into a tight beam, and the power of a transmission station increased, the waves can travel many times farther. Also, once the Tx waves leave the earths atmosphere, they travel without interference.

{edit}. Think of it like the range of a flashlight set to wide beam, aimed in the dark, through a light fog, compared to a stadium spotlight on tight beam focus.

Because cell towers transmit waves omni-directional (not focused), and are also relatively low powered transmissions. When a dish is used focus the transmission waves into a tight beam, and the power of a transmission station increased, the waves can travel many times farther. Also, once the Tx waves leave the earths atmosphere, they travel without interference.