Because there is a lack of trust.
Imagine you had beef with a company (OCP) that ran the police force because you didn’t get along with the executive chairman.
Now imagine the police came round to your house and demanded to search the property while you know you’ve done nothing wrong.
Imagine they then come out of the kitchen and started claiming the kettle you use is old enough to put your family and neighbours in mortal danger because the plug is a bit iffy.
If you start giving everyone free access, if someone really wants to find something, no matter the company, they will. It’s therefore better to take the pinch of not co-operating and let them produce any evidence they have themselves.