Because of this:
We reproduce 3 of the missions. This one is Edgar McElroy's.
They bombed the Yokosuka Shipyards where the Ryuho was under construction.
From there, like everyone else they headed west to China without enough fuel to make it. There was no significant damage inflicted but the attack stuck in the minds of the Japanese leadership and was instrumental in the decision to capture Midway. And at Midway, Japan lost it's edge.
If you wish, read about this mission here: http://www.doolittleraider.com/raiders/mcelroy.htm
Five of Doolittle's Raiders still live among us.