The are often things that will be apparent to a character, that would be unknown to the player. For a start, the character is often more intelligent than the player, but aside from that they have lived in that world all their life, whereas the player has only lived there a couple of hours. Sure, it will eventually come down to one or more dice rolls, but part of playing the game is trying to come up with the option that has the highest chance of success - i.e. the lowest DC. Otherwise, you might as well play Snakes and Ladders.