Why Is It?

The reason is two part.

One is pure ignorance. Not really knowing what or how to check it if you did.

The second is a biproduct of corporatism. I have a friend in who is field service in Simiconductor. They want that machine up and running ASAP. A down machine can cost the company upward of $50k and hour. He is literally not allowed time to trouble shoot the actual issue. Its throw parts at it off a best guess case scenario based on seeing this problem before. Dealerships are the same way. Jobs are book rate so if they can guess and throw the right part at it and it works they move on and make more.