3. You don't have the environment an actual child would have when learning their first language, which has the crucial elements of: a) constantly needing to in

3. You don't have the environment an actual child would have when learning their first language, which has the crucial elements of: a) constantly needing to interact with native speakers in their language to accomplish goals (obtain food, get them to stop screaming at you), and b) those native speakers attempting to figure out what you want and telling how to say that. I suspect that the supposed greater ease with which children learn languages is mostly explained away once you control for those factors.