It'll take a lot for them to be very profitable. Look at Alice.com: a lot of hype a few years ago and now struggling. When you're talking about groceries, you're already in low margin market. Now add discounts? commissions? % cut? lower margins
It's a tough business, you're right. But the nice thing about partnering with existing, main-line grocery retailers is that there's capacity to scale more quickly than a site that's wholly direct-to-consumer. And there's no inventory or shipping to mess with in this model.