That's because you're assuming the stranger doesn't return the car. If your mechanic takes your car for a joyride after you give them the key for purposes of repairing your car, and a stranger steals my car when I'm not using it and brings it back before I notice it's missing, I don't understand why one is any different or worse than the other.

In my jurisdiction, a mechanic who takes a car for a joyride is committing a class A misdemeanor (unauthorized use of a vehicle in the third degree)

ref. https://codes.findlaw.com/ny/penal-law/pen-sect-165-05.html

In other jurisdictions (like, say, New Hampshire), that same case falls into the definition of theft.

http://www.gencourt.state.nh.us/rsa/html/LXII/637/637-9.htm