Although this may not be correct but it's just my opinion as you asked for. Well technically I have 2 reasons why, 1 opinionated and 1 logical.

My opinion is that Snake was testing Raiden he really didn't give a crap but he was just there to help you.

The logical reason would be that the game would be easier that way. Except he kicks ass and rules all hell at the end of the game but that's a different story...