It's not like all this data was randomly dumped at the AIs. For data sets to serve as good training materials they need contextual information so that the AI can discern patterns and replicate them when prompted.

We see this when you can literally prompt AIs with whose style you want it to emulate. Meaning that the data it was fed had such information.

Midjourney is facing extra backlash from artists after a spreadsheet was leaked containing a list of artist styles their AI was trained on. Meaning they can keep track of it and they trained the AI with those artists' works deliberately. They simply pretend this is impossible to figure out so that they might not be liable to seek permission and compensate the artists whose works were used.