If a human learns to code by reading other people's code, and then writes their own new code, should they have to attribute all the code they ever read?

Plagiarism is a concept from academia because in academia you rise through the ranks by publishing papers and getting citations. Using someone else's work but not citing them breaks that system.

The real world doesn't work like that: your value to the world is how much you improve it. It would not help the world if everyone were forced to account for all the shoulders they have stood on like academics do. Rather, it's sufficient to merely attribute your most substantial influences and leave it at that.