First off, this has nothing to do with Airsoft Gun Discussions: moved to General. Please post in the right section or you'll receive an infraction.
Secondly, why? Because it's fun and I like it. Why else?
Any sport/hobby has costs associated to it and as far as adult hobbies go, airsoft isn't all that expensive; I'd say mid range, and maybe even an inexpensive hobby if you consider a lot of the new low cost guns and gear that have come out in the past couple of years.