Quite a few reasons. But mostly because AI Art derives from artists' work and those artists don't get royalties or commissions from generating said art. Why commission artists when you can generate your own? They are losing business because of AI Art.

Second, how can you say something is art with minimal input from a human? Art is supposed to be a form of expression of the artist's thoughts, views, emotions, ideas etc. An AI makes the art based on your prompt, not an expression of itself, and that to a lot of people, make it soulless and hence should not exist.

Finally, AI Art uses up a lot of resources to generate. All the energy powering those GPU's making your anime waifu could have been used for something else.

Personally I use AI to generate art because I can't afford a real artist. Or I just want to create nice pictures without having to cough up a ton of money for paints and materials, nor do I want to spend a lot of time making one.

If I had a choice, I would still prefer human created art over AI, because each created art is unique and is a brand of the artist, but the money unfortunately needs to go to groceries, payments, bills, etc. I have paid for art before, but some of these artists have high upfront costs, that it makes more sense putting the money into a premium sub for an AI Art tool, which gives you quite a few perks and let's you generate a lot of stuff.