Bad workmanship

My car was done panel beating at this workshop. After the repairs were done , first the door was not properly working, door controls on the driver’s door not properly working and driver’s window had issues in closing. Took the car back to them and this was sorted.

When I thought all is well, then there was a sensor hanging on the right rear beneath the bumper. The car have some warnings . Took the car to Mercedes Benz then found out the AMG motor was not properly placed and the pins rusted due to water getting into them.

So this workshop is Mercedes approved , so how do they make such mistakes. Is it because they take too many vehicles and work in a rush ??