I get annoyed with heating engineers who fix a boiler in 4 minutes charge £90 and don't tell you what the( obviously simple ) problem is.
To be fair, I am sure you are very sensible, but from experience with the great british public, if gas engineers went round telling people what was wrong with their boilers it wouldn't be long before some idiot caused an explosion trying to fix something themselves, and then they or their insurers would try to blame the engineer.

I can see why they don't.