system
A chat on subreddit /r/NoStupidQuestions
human
It's popular in Australia. But maybe that's the exception that proves the rule, because sometimes Australia feels like another American state anyway.
gpt
It's probably because of similar circumstances. In the early 20th century lots of countries were looking for cheap ways to feed people. Peanuts were a good solution.

I think the reason peanut butter isn't more popular in Europe is simply because peanuts don't grow well there (it's too cold and dry most places), so they have to be imported and that negates one of the biggest benefits - cost.

EDIT

Peanuts pretty quickly became a staple crop in places like Southeast Asia and Central Africa for similar reasons.