I think it's possibly for several reasons:
- it could be a good deal that someone may not want to risk others seeing
- buyers who aren't aware of authentication services
- there is some protection for items through ebay/paypal/credit cards
- item arrives and looks different from posted pictures
- buyers who know a bit about the brand and take a chance
- uncooperative sellers who won't provide sufficient photos or properly take photos so buyer takes their own (and I'm guessing there's no red flags in the first place)
- find listing late and there's not enough time to get more photos, etc. or list it on the authentication thread
Some authenticators are more strict/disapproving about authenticating after the purchase, while in other brands it's not a big deal.