No, it is not more efficient to channel aid multilaterally, because that only involves more organizations. The more countries and organizations that are involved in extending aid, the more people have to spend time and money to oversee the program. It is more inefficient the more people that are involved. Bilateral aid is easier and can have more direction.
The American taxpayer likes to know where their tax money is being spent and would prefer that aid money is handled directly, than by the United Nations or other international groups. Corruption in some countries is responsible for the misappropriation of funds targeted for in-need groups and is a waste of the taxpayers' money, as well as the increase in administrative costs by having a "middle man". Direct aid assists in elevating the image of America abroad, and assures that assistance is going to the targeted groups.