...and because there was only enough fuel to make it to Tokyo but not a return flight.
The Hornet was spotted by Japanese fishing vessels which doubled as picket ships before they got to their planned launch position. The fishing vessel was sunk by the Hornet’s escorts, but not before it sent off a warning to Japan. The danger was not that the Japanese would anticipate an air raid, but that Japanese naval vessels, including submarines would attack one of the three U.S. aircraft carries in the Pacific.
Since there was no possibility of continuing to the planned launch point the dilemma faced by the commander was whether to launch immediately with the prospect of ditching in or near China of calling off the raid. There was probably not a single man who would have wanted to call it off.
The attack was militarily and economically insignificant in itself, but it was a great propaganda victory and morale boost for America and a major embarassment for the Japanese Navy. The Japanese response was a plan to seize the western Aleutian islands and Midway, which would have foreclosed the opportunity for a repeat performance. The Navy knew about the plans from intercepts and they could not counter both attacks. They understood that Midway was the more important objective and defended it. The result was probably the single most decisive naval battle in history, and decisive in a way that did not favor the Japanese.
So, by luring the Japanese Navy into an ambush, the Doolittle raiders accomplished more than anyone could have realistically ever have hoped.
That and the fact they could take off from the carrier but couldn’t land on it.
Additionally, there was no way to recover/land a B-25 on a carrier.