There are other factors that go into the legal determination of fair use, including the amount of the work used and the potential effect of the use on the economic market for that work.
It's not a correct interpretation of fair use to say that anything non-commercial is legit. (And, even then, in the case of corporate AI it is very commercial...)
https://fairuse.stanford.edu/overview/fair-use/four-factors/
But beyond that legal definition, let's talk about what seems "fair" in a basic human sense:
If you were a painter who who makes art for a living, does it really seem "fair" for someone else to take your work, without any sort of consent or compensation, and use it to produce other works that are in direct competition (for time, money and attention) with you? How is that "fair" in any sense of the word?
That sounds like exploitation to me.
Call me crazy, but I think the working people whose labor and creativity have made "AI" possible should be reaping the rewards of its existence, not the capitalists and oligarchs in control of the big tech companies.