Intel doesn't have a channel to get these products distributed. Google at least has google.com (billions of users). Amazon has amazon.com (100Ms of users). Samsung is an established OEM with vast sales channels in retail. Intel would fail going up against these guys. The distribution channel for silicon and boards is very different than the distribution channel for systems.
I think mas's explanation was probably best - the OEMs have no idea what stuff is going to sell, so they're building each model cautiously and seeing what the customers like.