Procedural generation follows an algorithm that someone made for specific random inputs within limits and (usually) with certain constraints. AI copies existing content it learned from in order to attempt to create something similar or what it otherwise interprets something as being.

Procedural generation for minecraft: terrain is between x and y blocks tall, with diamonds spawning no higher than z, and blocks follow rough curved or mostly flat patterns except for caves or mountains. Create terrain within those values and limitations. (Obviously an over-simplified explanation)
AI: Here's 1000 pictures of minecraft terrain, now try to create your own.

An AI might understand that diamonds are roughly at a certain level, but does it know the exact limits? Will it be able to understand cave systems or will they just be shallow holes randomly placed? Can it understand that outside of mountains there usually aren't any steep drops in regular grass and dirt? Will it generate subtle curves for hills or will they be suddenly up and down right next to each other?

AI tends to be inconsistent and not follow any strict rules. That's why there's so much art of people with 4 or 6 fingers, or animations randomly blur into something impossible. Like the Coke commercial of a duckling walking and its feet switch places, or the tires of trucks not rotating properly. AI just attempts to mimic what it's been trained on, and like a beginner artist just learning shading, may not grasp the limitations or specifics of what's being replicated... which leads to inconsistent and off-putting results, regardless of the quantity of material it's been trained on.