AI content is constantly rejected, why?

The refusal is "quality issues". So you should check your assets at 100% for quality issues. The text you name is stock text. That will be shown for all refusals of this kind. That may or may not be true. Image size is not a quality sign. Other sock agencies may have a differen quality threshold. That's not an argument.

This said: your images are indeed soft in focus and expose some noise.

 @daniellei4510 @Jill_C  did mention the conceptual error with the strawberries and the other images. generative AI is very dumb, despite being called AI. Never forget that and check for logic errors.Here are some: 

(glued to the glass or the straw?)

Ice is lighter than water: 

In addition all assets have exposure issues and they are either missing the whites or the blacks or both, when you check the histogram. 

I suggest you analyze your assets with a more critical eye, improve the upscaling and check the "exposure" and noise. Even if the moderator would not have a critical eye (and I suspect that the original refusal reason is indeed the softness), buyers will complain, if they do not see the conceptual errors right away and will avoid your assets. 

Blueberries look like glass and are shiny and irregularly drawn.

Chocolate covered strawberries are dipped from the top, not the bottom. Lose the stem and finish the decorative line of white chocolate in the middle.

Straws do not have lettuce or whatever that is in the middle of the green one. The fruit looks like it has been stepped on, and the ice cubes are poorly drawn.

The blueberries look like solid marbles. The fruit in general is poorly drawn.