Two main reasons.
#1. Power.
The more power you have the less gear advantage you need. Gears are just there to give the engine more of an advantage.
#2. Transmissions with a lot of gears.
Final ratio's are where it's at now days. As in the racing game, we did not care what the rear gear was all long as the math came out right for our final drive. Sometimes we would run 1 to 1 and a 650 rear gear and sometimes we would run the tranny in 2 gear and use 389 rear gears. It made no difference because in the end it was all the same.