That is because most SC don't have panel and body work personnel. When I had a Honda there was a rust spot on the door. Send back to SC they in turn have to send to their contracted body shop for respray.In the case of my friends GTI, a bike sideswiped her side mirror and the plastic panel popped out. She sent it back to Wearnes for repair and they couldn't fit it back. Fitted it eventually but she discovered the car had a few hundred KM added to the mileage. Joyriders? 100km is somewhat acceptable but a few hundred is unacceptable. It was something like 300km+ total.
2nd time sent back to change drive shaft based on service records, the mechs didn't notice a damaged sidewall in the 18" tyres and the somewhat flat tyre and didn't inform my friend. She only discovered the problem after driving back from the center and heard a strange droning sound. By then the tyre was unrecoverable.
Other times they didn't fit panels properly causing rattling, didn't screw properly causing loud banging noise when the bolt came loose while driving that shocked my friend and thought something had broken in the engine bay.
All sorts of things.
Lately my wife's Tiggo had door alignment problem, mention to SA during one of her service, was told body shop expert not in until the following month (They shifted workshop). So next round only got it done.
As for joy ride, well one of the SA in one SC I know was sack because he took customer car for a ride, so it all depends on the Administration of the SC.