Because you can just look into it and see if it's what you sent fof production, and if not and the word gets out you are done as a fab. Fab business is about trust. You also should trust that your design isn't leaked to the competition.

It's very common to xray the dies, especially for debugging. Also common is to etch it layer by layer, take photos and rebuild the circuit schematic, mainly for reverse engineering but I've seen companies doing it to their own dies too.

Things get more blurry at the board level, the combinations of suppliers and service providers are endless.