You can trust builders who will fly on their own airplane.

If the mechanic is getting on the plane with you, you can trust them to make sure it works.

They have a deep structural ownership over the problem, because it is tied to their continued existence.

They have the incentive to go above and beyond and be curious about parts that might not work.

Compare that to someone who doesn't feel ownership over the problem.

For example, an electrician at the factory who says "Well, I did the electrical work properly" but doesn't trust the rest of the production process to produce a result they'd actually fly on.

This is also part of the logic of eating your own dogfood in product development.