Because lots of people like to shop together with friends, and most ecommerce sites don't provide a good or even any experience for social shopping. Facebook or someone will eventually get this right. Pinterest is a down that path.
OTOH, I don't see a need for Facebook to necessarily be involved, since I don't think people shop with all their - just a few. Not being a social shopper though, I can't comment much more than that.
Even if I wanted to shop online with somebody else, I would do it on the store's website not on Facebook. I wouldn't mind if Facebook powered the interaction behind the scenes, but I don't feel good about shopping on Facebook.
OTOH, I don't see a need for Facebook to necessarily be involved, since I don't think people shop with all their - just a few. Not being a social shopper though, I can't comment much more than that.