Because what is the point if something is distributed in a restrictive license, can't be preserved and then gets lost to time? Also, licensing is to avoid distribution, modification or outright copying by competitors; preservation is completely orthogonal to those concerns. It is to avoid losing a piece of craft to the sands of time. There is no reason laws should have power over anything in perpetuity.
As seen in other spaces, pirates ignoring the "law" will provide the greatest service to humanity.
Laws are simply rules chosen and enforced by a given society. Having power over things is what they do. (Also, "in perpetuity" seems untrue, as all copyright expires eventually.)
You clearly disagree with the laws (and I'm inclined to support you there), but what is special about preservation that it should automatically override the will of society? Nearly all the combined work of humanity has been "lost to time," and society seems pretty okay with that.
That argument is almost entirely about the length of copyright, and you're dismissing that with a quick "eventually". It's not about trying to "override" the intent of copyright.
Also copyright has a clear purpose, and the purpose is to promote culture and science, not to help things get lost. When works that people care about get lost, that's a flaw not a feature.