A human artist keeps state :). They keep it between drawing sessions, and more importantly, they keep very detailed state - their imagination or interpretation of what the thing (house, grizzled detective, etc.) is.

Most models people currently use don't keep state between invocations, and whatever interpretation they make from provided context (e.g. reference image, previous frame) is surface level and doesn't translate well to output. This is akin to giving each panel in a comic to a different artist, and also telling them to sketch it out by their gut, without any deep analysis of prior work. It's a big limitation, alright, but researchers and practitioners are actively working to overcome it.

(Same applies to LLMs, too.)