Maybe because most works generate revenue, if at all, in the few years immediately following first publication, so a period based off that is all that's necessa

Maybe because most works generate revenue, if at all, in the few years immediately following first publication, so a period based off that is all that's necessary to encourage creation? Maybe because if a copyright holder has a family and dies 6 months after creation of the work it seems wrong to prevent the family from profiting off the work? Maybe because when limits on periods are required in the law there is often no absolute reason to prefer one of several good candidate periods over another, so the ultimate decision is always in some sense arbitrary? Note that in the U.S. patents are generally granted for a period of 20 years. Why do copyrightable works require or justify more?