I thinkk it ia a weight of the bike and the torque the engine puts out calculation, power vs. weight. For isnstance the Sachs Madass 125 is just a four speed also. The Ninja 25Or has a six speed but its power comes on at pretty high revs and it is a heavier bike than a GROM?
The fewer the gears the lighter and less expensive the bike.
The GROM isn't set up for highway speeds to start with so it lo looks like they gave it just enough gear for about 65 mph with a light load.
A SYM Symba has ail four speed with an automatic clutch but that is a 100 cc engine on a 200 pound bike. While people over load those things it really doesn't do well with over 150 pounds of rider and a six pack of beer. It is just an updated version of a Honda Super Cub.