Because it's fun!
Figuring out secrets and hidden tricks is entertaining, like a big secret puzzle. Having absolutely everything laid out and ready for you is kinda boring. I for one love the extra meta challenges. It's why I love the original Zelda. You had to do some work on your own. You didn't always have maps, and the ones you did have weren't very clear. You had no direction beyond a few subtle hints. You had to figure things out and think about them.
It feels to me that a lot of games these days make it far too easy. Not a lot of thinking involved, you just move from point A to point B like the NPC told you.
But whatever, I'm being nostalgic.
