Sadly I think it's more that there isn't really a standard way to buy books and other media in bulk at the scale of which AI training usually requires. So the companies realise they can save both time and money in just pirating after calculating the fine risk. Its just a bonus that they usually get away with it and that the fines would likely be cheaper than a legit transaction. But i do think it's the bulk data packaging that makes piracy look more attractive to them at the get-go.

Heck, even video game publishers often source their roms for their official re-releases from pirated copies because pirates are better at preserving data and keeping it in a nice friendly format. Easier to search for it on the web and download it then it is too goo into their own archives and rip it themselves, if they even still have original copies, cause they sure as hell didn't keep their source code.