It took us thousands upon thousands of years to arrive at this simple idea because it's a blow to our egos and to primate hierarchy and its pretensions of authority. If I am big monkey, I must be right.
It's just that: wanting to have more accurate beliefs. Believing that this is worthwhile even if it's hard to push against human nature of habitually "choosing" beliefs.
My idea is that through first principles and made up probabilities, they can reach any conclusion they want. But there's a veil of authority: "I'm a rationalist, therefore I arrived this conclusion, surely not from rationalization, but from first principles, probabilities and logic! It doesn't matter if it sounds insane, because it uses first principles, probabilities and logic!".
Robinson Crusoe has no use for rationalization. Him rationalizing the berry he ate is not poisonous, or that the shack he built will hold up in the storm is outright dangerous, and definitely useless.