I try to ask myself this often. But I think because Vintage clothes is sometimes, usually, uniquely made for one person. You can find an outfit that fits you better and is more body flattering than something off the rack that is made to fit many bodies. At the same time, you might find something you like so much you could scream, and then it doesnt fit you right or fit you at all.
I agree with the hunt part on some aspect. I will blend this answer with another one. The possibility of finding great designer pieces that someone else has tossed away is a huge rush. I know a lot of record collectors, and they can sympathize.