they are used. For an Ebike, its used as a hub motor. lace the spokes to the outside of the motor, and you have a direct drive.
The are being tested by a few companies for cars as well, but the efficancy band is smaller than a conventional geared drive. thats fine for a bike with a narrow speed range, or power tools, pump motors, forklifts, ect... but for a car, there are other, more efficent ways to make them work