The common hand shapes were arrived at via evolution, meaning they are the shape that most encouraged survival, and doing well enough for oneself to get opportunities to reproduce.

If the longest fingers were on the outside, they could get bent back or hyperextended if the short middle finger couldn't provide support. With the shorter ones on the outside, a person losing their grip would lose it one finger at a time. When the long middle finger loses grip on something, you're only holding on with one side of your hand so there should be more room to wriggle free.

Also, the traditional ratio of finger lengths allows a person to make a fist comfortably. So, the digits are typicallly the correct length to be curled up comfortably into a closed hand.