Property in general is a social construct, not just intellectual property. When I say that something is my property, that refers to social/legal recognition of my rights over that thing. Even if that thing is tangible, there is still no black box experiment that can determine whether that thing is my property or not.
That is true; however, we cannot remove a tangible thing without it being obvious that it's lacking and unavailable to the owner. As a result, more people are on board with the tangible property social construct.
Intellectual property is more like, you and I have some tangible property, and I'm not allowed to tweak my property into the a particular configuration that is exhibited by your property (or equivalent forms), without permission.
That situation also defines privacy, and also secrecy.
I suspect more people are on board with caring about privacy and secrecy, than caring about copyright.
If physics cannot detect it, how real is, it, really?
If it's just a social construct, then it's only real to the extent that people agree that it's real, and it's real only to those people who agree.