Just imagine how much work they'd have to put into it, with the price rising and sinking by hundreds of dollars within weeks. Not to mention all the decimals people would have to deal with for cheaper items.
Overstock got it going in no time with the help of Coinbase. Coinbase can convert bitcoins immediately back into dollars, thus shielding the merchant entirely from price volatility (although many merchants opt to keep some portion of the proceeds as bitcoin as an investment).