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.

This is just the way starch works. And pasta is made of starch.

In raw pasta, you have densely packed starch molecules. When you cook it, the water seeps into the starch molecules on the pasta surface, and in the presense of high temperature swells them, softening the pasta. This is called gelling of the starch. It happens in the pot while you cook the pasta.

But for true gelling to happen, you need lots of water in your starch. And water only penetrates starch slowly. It is quicker in pasta (which is made of milled flour) as in whole grains (e.g. rice cooking), but it water penetration is still quite incomplete after typical pasta cooking times. It is high on the outside, but low on the inside.

Given time, the water expands through the pasta. If you drop a single drop of dye on a cotton towel and wait, you will see what I mean. So when you leave the pasta, the water is traveling from the hydrated surface layers into the dry core. This means that the outer layer is left without the water, and it either goes dry if left without sauce (never seen it? A heap of pasta will stay moist, but a single strand will go back to hard in a night), or it sucks in the moisture from the sauche. Since a sauce is mostly water, it is practically absorbed into the pasta.

This was the explanation for "what causes" the absorption. As for a solution of your problem, it is very difficult. It is like asking "I poured water over flour, how can I keep the puddle of water from being absorbed". I guess that you could try oiling the pasta before you apply the sauce, or other tricks. But none will work well. The sauce won't cling to waterproof sealed pasta. Maybe you could try presoaking the pasta before cooking them, but you'll get such limp pasta that they won't be worth eating (if they don't dissolve first). The only thing which works is not to store pasta and sauce together. Keep the covered pasta in a bowl in the fridge, the sauce in a second bowl, and mix before eating. If you want the sauce to really seep in, you can use a small part of it on the pasta immediately, knowing that it will disappear, and add the rest when you eat.

When I make my cold tuna pasta salad, I always make up the dressing first and add it to cold pasta, never hot or even warm.

To avoid having a "dry" salad I add whipping cream to the mayo, sometimes sour cream. Always stays nice and moist!

I add some whipping cream to rinsed pasta. Mix and refrigerate for a couple of hours. Mix every hour. Mix dressing. Your salad will be nice and moist.

How about an ice bath for the pasta before adding sauce. If the sauce includes olive oil it should stay moist