What makes for good alt text depends on the author's intent by including the image, an LLM can't know that.

Authors can often use an LLM to save time by outputting an initial draft an author can fix.

Readers can also use their own LLM as a supplement, to ask it questions about details in an image that wouldn't be in the alt text. LLMs will get things wrong but are generally good enough for such supplementary use.