To achieve what we're talking about (wireless with a switch), they'd be building a wireless access point and a network switch into the processor. It's a lot of work compared to just slapping an ethernet module under the hood, and it will then go unused for folks with no home network (who thus aren't using the ethernet connectivity on any of their equipment anyway) and folks who already have wired or wireless connections in place. When you can get something like this for $40, it's probably hard to justify the expense of building that into a surround processor. There's also the potential headache and support cost associated with providing an interface for configuring the wireless access point (encryption and the rest) through the receiver's user interface and fielding tech support calls related to it.

I guess that if it were me designing it, I'd rather let the customer get whatever they need to best serve their network than try to build a solution in and then support it. Of course, I also pulled four network cables through my attic to my entertainment center a couple of years ago, so my perspective may be clouded by that...