Well, the premise of your question is off. You'll be pleased to know that Memorial Day was created in the wake of the Civil War to commemorate the fallen (Called Decoration Day at the time). Its only in the 20th century that it has become a more general purpose holiday for all military dead. I won't pretend like I can fill you in on the details of its creation and transformation from ACW to all-purpose holiday, but that as to the premise of your question, we do celebrate it, in a sense. The better question (which I don't really have a great answer for), might be "What was the genesis behind Decoration Day, and how did it turn into the more generally themed Memorial Day?"

Similarly, Veterans Day was created as a commemoration of World War I (Then called Armistice Day), but morphed into a commemoration of all military veterans.