Because not all orders turn out the may hope. Some you just don't like, and some arrive DOA. It's easier to take it to a local store.
And some items break after the 30 day is over and you are responsible for shipping the items to the factory repair shop, sometimes more than once. If you bought them from Best Buy, you just have to take them to Best Buy and they will handle the repair for you (ship items for you, etc.).
Imagine paying the same price for a Denon X4000. If it malfunctions in 12 months and still under the 3 YR warranty, all you have to do is take it back to Best Buy and they will take care of it for you. If you buy from Amazon, you have to contact Denon and pay shipping $60 w/ insurance to ship to Denon. Then let's say you get the X4000 back. Then something else breaks in another 6 months.
Well, okay, maybe I should use Onkyo as an example. In which case, you better buy the extra extended warranty.