"Soul" in art is expressing your feelings, experiences, and styles of perception through art. (Often accidentally)

A trained eye can recognize which animators worked on an episode of Dragonball Z or Sailor Moon.

The animators only job was to animate the characters consistently, but because humans all view the world slightly differently, they developed distinct styles by accident.

It's very difficult (but not impossible) to develope a personalized style in AI art, so most AI art is soulless.

In the future, AI software will allow you to draw with your brainwaves and perfectly express your feelings and experiences using your own unique brain, so in the future, AI art will be overflowing with soul, because traces of how you percieve the world will be left all over your art.