Because LLM's don't "use" software like humans do. LLM's aren't trying to move thumbs around on a touchscreen screen, or push a mouse to click things. LLM's don't get annoyed when they have to wait ~30 seconds for something to load. LLM's don't have good aesthetic sense to tell when a font selection doesn't work, or a UI is too crowded / buggy. LLM's can't tell when an icon is too small to be seen.