For a couple of reasons.
Number 1, there is collector value in even some "junk" silver coins, stuff like common Morgan and Peace dollars will always, and has always, had a premium over their bullion value.
Number 2, there is a large disconnect between the paper silver price (the spot price) and what you can get physical bullion at. For example, the local coin shop near me is selling generic bars/rounds at $26, silver Eagles range from $28-30 depending on how much they have in stock.
Number 3, US junk silver is more recognized than a generic .999 fine silver bar/round. That is, in a SHTF scenario, it is easy to know if a US quarter or dime is silver because the average person knows what they look like and its a lot harder to fake than a bar which anyone can stamp .999 in even if its plated.
Some of the eBay prices look really high until you go to your local bullion shop and realize that they want the same, or more for the same product and, although not as much today as several weeks ago, a lot of the online bullion shops have (or at least had a couple of weeks ago) a 1-2 week delay before they fill your order.
