The law calls some thing "property" because they are property. Their status as "property" predates their legal protection as such.
But in the case of both land, copyright, and patent, it is the other way around: to the extent that these things are "property" at all, they are "property" only because the law, for pragmatic policy reasons, says they are. Neither land nor copyrights were "property" in the English legal tradition until fairly recent centuries. In the middle ages copyrights didn't exist, and land was not "owned" as such. Rather, certain rights to certain lands could be owned and transferred, certain other rights were in common, certain others were often reserved for the landlord's social superior. Even in modern times the word "property" doesn't apply to land as well as it applies to personal goods.
The public policy question, of how long copyrights should last and how broad their scope should be, is not solved simply by repeating the word "property" as some sort of magic spell. Even if we overlook the enormous difference between your-pocketknife-as-your-property and your-copyright-as-your-property, we are still stuck with the question: what is the best way to regulate this "property" for the common good ? My answer is: in the case of copyrights, they should be kept short-lived. Everything good the copyright monopoly does, it usually does within 50 years or so if it ever will. Everything bad that it does, such as restraining creativity and acting as a tool of censorship, can be made worse by long durations.