Because he had never known any different, and neither had anyone else around him, no one had anything to base their perceptions on, which meant that they had no reason to think twice about the fact that he had no clothes. As it is now the norm for us to be fully dressed at almost all times, those who are not are questioned, if not frowned upon as not conforming with the ways of society. In a time where the way of society was to wear no clothes, and nothing else was known, why would you ever care about the possibility of anything different?