Accepted payment for out of stock item, 3 weeks for refund

I ordered (and paid for) a product, only to be informed on the day of expected delivery that it was out of stock and that it would take up to three weeks for me to receive my refund. Why would you accept payment for something that is out of stock? Not only did I not get what I needed, I now have to wait for up to three week sbefore I can get my money back, and try to find it elsewhere.