Why do cellular telephone systems only need seven sets of frequencies in metropolitan areas?

For frequency reuse, telephone systems use 7 sets of frequencies in metropolitan areas.

Explanation:

As the population is high in metropolitan cities it requires faster reuse of frequencies so that everyone within the network region is being able to use their cellular devices. It can be imagined in the form of a hexagon with 6 points and one at the center enabling faster reuse of free frequencies.