But there’s obviously Cub types that will fly at glider speeds so why don’t they have squat switches.

Trying to apply logic here is risky, but maybe because the STC doesn't ask for it because the airplane is not intended to fly at zero groundspeed and so emitting ground mode while flying would be inadvertent.

If the glider is intended for wave and/or not experimental, then the STC would ask for the switch and zero groundspeed in the air would be expected. So being able to convince an investigator that you did a good faith effort in emitting the right signal "during the appropriate phase of flight" would lead one to the extra switch.