Your disbelief doesn’t mean it’s not happening. The data sources that go into AIs are indeed curated selectively. Honestly, what do you think happens, a webcrawler is told to just “go nuts” and whatever random data it spits out gets fed right in? Trainers pick their sources carefully. They deduplicate it, they format it, they do a lot of work on it.

Perfection is not required. Human error is fine in manageable amounts.