For car mechanics, I know that replacing the oil is a much simpler job than replacing the engine

On the other hand, if I come in complaining that the car is making funny noises, it's a lot harder to see whether this is an easy or hard problem. Another example, I come in for a routine inspection and he tells me that some part I've never heard of needs replacing and it's going to be expensive. I have no way to check short of going to a different mechanic and then some figuring out who to trust.