I remember when we got our new washing machine (clothes), and I was like, where is all the water? And the answer was, you don't need to fill the whole thing up with water to get clothes clean.
Now, I don't think it rinses as well as it could, and it seems like more water would help there, but, you know, these companies have really smart engineers working for them that do things like figure out exactly how much rinsing has occurred and stuff like that, so my opinion isn't really all that meaningful in that regard.
The first thing a dishwasher does is to assess the contents to determine how much water is going to be needed. And it uses that, instead of wasting water.