They don't, and I agree that it is flawed. But perhaps flawed is too strong of a word, and it's just that ebay auctions don't tend to mimic real-life auctions (and maybe there is a reason for the discrete time limits, ie sellers need to know exactly when items are ending; buyers are often time limited sometimes as well: 7 days often seems like an eternity anyways). Many of the up-and-coming penny auction sites include this feature, as it is much more necessary to their business model.

I think Ebay realized at some point that marketplaces are more profitable than auctions and adjusted their model accordingly. Now they're in this limbo between an auction site and a marketplace site but not great at either. They can afford this of course because they're a monopoly, but if I really want the experience of one or the other I'll use a service with clearer focus.