Human programmers at least can tell you where they got a snippet they copied, whether it was in the docs, stack overflow or elsewhere, and you can try to keep attribution if you care about compliance. Not only that, but most of our skills are related to designing stuff and recognizing which pattern to use, the specific implementation isn't necessary the same unless we go look for whatever we saw in the past, as our memories don't just record everything and repeat it word by word. And after picking up a new language or framework I only need to look around when using a third party library or some API I'm less familiar with, or when something breaks.