I do it for the same resin you do it. To me making costumes keeps me from doing stupid stuff like drinking and partying. I love making them and I love the reaction I get from it. Im also an introverted person and it does help me to get out more. And yes I would say it is a form of artistic expression, because in a why you are sculpting something out of foam, fabric, fiberglass, extra, into something different.
P.S. sorry for spelling errors.