I understand the sentiment and partly agree. But also, the original has not gone anywhere. You're free to accumulate knowledge in the old way just as before. So maybe it's not theft of knowledge that we should be angry about, it's something else harder to define.
Lots of the original content is no longer available. Bots kill sites, AI kills monetization - both results in the original material disappearing.
It's not the fact that they scraped the knowledge and used it to train a model. It's the fact they're trying so desperately to corner the market so that we're all reliant on them and only them, and have no means to free ourselves.
I've heard people say "theft" of intellectual property a lot. Also stealing an idea is common parlance. Maybe it's regional or something but I hear "theft" or similar used all the time for things other than physical goods that you lose access to.
The original might no longer be there as the site might have already shut down due to AI scrapper bot overload. Or the original was a book Anthropic scanned and then shredded. Or the artist stopped publishing their works or doing art all together after all their creations were ingested into the model blob without their permission.
It is really insane to compare individuals copying data to big corporations parasiting on the Internet.
We do have some unique carve outs already for what we consider intellectual theft e.g. trade secrets. In this case the original artifacts might remain, but admitting the market that created them could go extinct is enough definition to be infringement at least. Fair use has been really resilient in these training cases so far but it is pretty damning to admit a negative market effect and that you're a direct substitute (see Warhol v Goldsmith recently).