I agree that tying copyrights to lifetimes doesn’t make sense, because it’s logical for copyrights to be inherited and the law shouldn’t benefit long‐lived authors over short‐lived authors. I’m not fond of the reasoning in your comment though—by my reading there’s nothing stopping it from being used to extend copyright to “forever less one day.”

US copyright law at its inception was intended to provide a balance, incentivizing authors to create new works without infringing too far on people’s inherent right to copy and share ideas they haven’t necessarily created. See Madison (who wrote the original Copyright Clause):

“But grants of this sort can be justified in very peculiar cases only, if at all; the danger being very great that the good resulting from the operation of the monopoly, will be overbalanced by the evil effect of the precedent; and it being not impossible that the monopoly itself, in its original operation, may produce more evil than good.”