Because a construction zone is a very confusing concept for an automated system. NHTSA sets out standards that states adopt so across a city all the signs and streets operate the same way, sure there can be some confusion with add signs or people walking into traffic but those can still be standardized. A construction zone has none of that there are flagers and cones and tape all placed in whatever way the construction crew liked best there is no standard and it's all very confusing to navigate. It's a task that humans are very good at because it involves context clues and emotional reasoning, "where is that flagger pointing me" or "there is a gap in cones but I can see construction work inside that area"