Because there are different types of riders. Most riders have one thing in common. They need a light gear for uphills and a heavy gear for high speed descents. The difference is the gradation of the gears.
A touring cyclist appreciates 18 finely spaced gears and a wide range finely graded gears > P1.18.
The recreational cyclist / mountain biker usually prefers medium gear jumps > P1.12 or C1.12.
The e-biker does well with 9 broadly spaced gears and a lower overall gear ratio thanks to the support of the motor. ->C1.9
Of course the needs are different and depending on individual factors a transmission with more or less gears can be useful.