Because its basically impossible to completely and accurately attribute the origin of all your knowledge. And it is impossible to verify that the source you think is the originator of your knowledge is the original creator of that knowledge. Odds are they learned it from someone else. It really doesn't matter, at all.

> This whole discussion is like if you train an AI to pick apples from a supermarket and leave them on the street waiting for someone else to take them home, and pretending that nobody is stealing anything.

No, because in this case the supermarket has lost apples. This is more like accusing street performers singing popular songs without permission of the songwriter of being thieves. Or an engineer studying a bridge and leveraging techniques used in that bridge.