It’s not even that they’re slaves, they seem to be happy with that.
But they really want to fly (click on them, do side quest with kowalskos), everyone around them are flying. They’d probably would do their job better if they could fly. And they can’t
I’m assuming because they have hands at the ends of their stubby wing-arms so they can do work, and are too heavy (being muscular so they can…you know, do work) for their stubby wings to lift them off the ground. Of course, you’d think in a universe of magic that wouldn’t matter.
Bah, they are souless automatons created for the sole purpose of making their master’s life easier. Do you worry about your dishwasher’s secret dreams of flight?