> Go for a clean, unfussy, bright layout with a bold striking spring-themed graphic. Avoid pastel/airbrush/oil style art or images of people.

This seems to be the crux of the issue. It's not that the AI posters were bad, but has all the hallmarks of low effort. And if the organizer can't be arsed to put effort in their poster, why should anyone think it will be an event worth attending? The quoted prompt is not something someone can think of off the cuff. It requires someone who have seen enough posters to know what they would like and what not to put in. That takes effort. It's called AI slop not because it looks bad (though I do think it is), but no effort is put into them.