"Secret" doesn't mean that it is not there. Is there not pills in a bottle if we don't hear them shack around in the bottle? "Behind" would just mean blocked by like the bottle. For another question if you stick your hand into a cardboard box; is it not there?
The "reason why" and the "reason that", is a very easy on to answer. The "reason why" we do something is because it is knowledge, something we are taught. The "reason that" we do something is because it is our wisdom. Wisdom is something we learn, but not like knowledge, because knowledge is what we know about something. Wisdom is what we have had in our lives. We have never seen or anything about a dinosaur, but we have knowledge of them.