Why is life+70 so standard? Even life+50 seems excessive. I can understand life, I can understand protecting copyright for a number of years after death so some

Why is life+70 so standard? Even life+50 seems excessive. I can understand life, I can understand protecting copyright for a number of years after death so something written just before an author's death still benefits their family. But to preserver the copyright of works written when they were young, for literal generations after their death, that's just plain ridiculous. Also, life shouldn't even matter once the copyright is not in the hands of the original author anymore. Just give copyright a set length from time of publication (30, 50, or even 70 if necessary), unless it's held by the original author, in which case it's the maximum of life or that number. That already seems way more than necessary. Anything more than that is just monopolisation of culture and has nothing to do with protecting the interests of the author or the initial publisher.