Convenience. Scooters are all about convenience, and ATGATT isn't convenient. The lower speed also implies lower risk, because, face it, we're hard wired to equate speed with risk. Lower apparent risk, lower need for ATGATT, both to the rider and to onlookers.
Not asked, but implied, is why don't bicyclists go ATGATT. Easy.
If they did, they'd die.
You'll see motorcyclists out there in 45 degree weather with electrics, passing the bicyclist wearing just shorts, maybe a long sleeve jersey, gloves, helmet and shoes. Maybe it's 25 degrees, and the cyclist has long riding pants and a light jacket, and there's steam coming off their head. Bicyclists generate a great deal of heat.
Okay, so maybe they wouldn't die, but the fact is ATGATT isn't flexible enough, weighs too much, and doesn't breathe nearly well enough to be practical for cyclists.