Because it's light and fun. AR-15s are cheap and customizable too. Having lots of different guns at a certain point has zero to do with need, if it ever did. Many people just like to shoot guns, but far fewer say that.
I agree. Heck, it is fun to shoot stuff. I just don't understand why and how this turned into such a powerful political movement. My fun turns out to contribute to mass killing of children. Maybe my fun doesn't justify such lock-step ironclad opposition to additional gun control measures. If an "assault rifle" ban saved one life, and I can keep my .22 semi-auto pistol and my M1, what is the big deal? The NRA makes it seem like DEM gun control measures are an attempt to subvert a whole way of life, which is total nonsense.
Maybe we should focus more attention on why some children are being hurt so badly that they lash out violently, and less on what weapon they use when they do.