Because pirated versions will be running a VLK license while there is no VLK subscription on file or run a KMS software to fake the authentication of licenses.
Or in some cases, just run pure unlicensed and Windows will tell you on the desktop itself that the copy is unlicensed.
If inspected, you have to prove you have the correct licenses.
In some cases you'll be allowed to just buy the licenses there and then, but if you've been running dozens of unlicensed copies or dozens of straight up illegal copies (with faked/cracked/stolen licenses), they'll put the hammer down and you'll be audited in detail to the point they'll end up billing AND fining you for every piece of software you've used in your entire history.