problem is they pay less than what motorcycle taxi drivers used to make a few years ago before they became so popular. It's probably about the same now, if you drive for Grab or drive a proper "win" motorcycle taxi, you'll maybe make 300-400 per day, which is peanuts. Before Grab et al, they could make more like 600-800 a day (though it varies based on the location). I know because my girlfriend used to ride one and she's friends with lots of drivers.
So it all boils down to the fact there's no recourse. Grab is taking a hefty cut and you can't beat them or join them without accepting the same lower wage.