The same point could be made about a dentist or roofer. The dentist is essentially working on flat rate when they quote a price for a cap and the same applies to a roofer reshingles a house at X dollars per square.
My brothers in law are in the masonry business and I think they quote their brick jobs per 1000.
They get the same amount of money no matter how slow or how fast the job goes.
Mechanics also spend a godawful lot of time (for free) test driving cars, waiting on parts, doing the paperwork shuffle, or waiting on an authorization to fix the car because no one is answering the phone at the customer’s house or they “want to think about it”. The latter effectively ties up a rack and shop space; especially irritating when a car had to be pushed in or is partially torn down.
The latter often leads to rounding the crew up and pushing it back outside while someone makes up their mind which means the other flat rate mechanics are earning zero while pushing.