At least in this case the US copied Europe with it's term extensions (Disney certainly weren't unhappy about it but the notion that Disney drove this is a bit of a myth). And Europe set that term length when the EU unified terms because it set it at the longest length from the member nations at the time, which was Germany's life+70 years.
It's pretty interesting but the current global norm for copyright duration is, due to circumstance, almost entirely based on German norms.
Why should every government copy what US does?