Because cost of original training >> cost of distilling. It's the same thing that happens with Chinese knockoffs of physical products - it takes a lot of money and R&D time to design a new product, but it's basically free to buy the product, reverse engineer it, and resell it. All the data they originally trained on was available for free on the internet. If the original work was so valuable, it shouldn't be up on the internet for free in the first place imo.