It's like if you toss all ingredients into a big pot and cook them until they're indistinguishable. That's why people call it AI slop. It basically works on the same principle.

serious answer: they don't know how to prompt. not that knowing how to prompt makes it look better, but it can at least make it look different. like how google used to give you more specific results if you formulated the query in a certain way.

I lot of these people just want a menu board. One of the things about being a designer is you have to have some taste and understanding of the field. This is just business owners who are trying to check a box and don't actually care. This is compounded by the fact that few small restaurant owners come at it from the 'restauranteur' side of the spectrum and usually come from the food side. This means design of anything is pretty far down the list.

It’s inherently uncreative and it’s even more likely to produce same-y material when you 1. Don’t give it a specific art direction and 2. Use the same generic models everybody else is using (a result of Googling, “AI art generator”.)

They put zero effort into it. Given instructions AI could make posters in different styles. But that would mean you know how to describe the exact style you want. You need to have some knowledge about design, for example by being a designer.