Why is my macaroni salad absorbing all the liquid?
Pasta absorbs liquids, because it is made of starch, and starch absorbs water.  Tossing the pasta with oil will reduce the absorption, by coating them in a water-repellent layer. 

Beyond this, the solution is simple: don't drain as much water from the cooked pasta, and add more dressing.  There's a limit to how much moisture pasta will absorb.