Because it is worth repeating. The more consoles that stick around the better (I would be happy if Sega was still around, for instance). What some people on these forums are asking is for a console maker to basically hang their $600 device out to dry, or hang themselves out to dry. It's a fine balancing act and the scales aren't tipped in the favour of $800 boxes being sold for $400 for this upcoming generation. Nor will you have them "waiting for better/smaller/cooler" hardware years after other console makers release their product, at their own behest.
If you want the best visuals, you won't find them on a (comparatively) tiny set top box in front of your television. What you will find is something that provides great visuals for a great price with a great hook (i.e. a much better Kinect), not something that's bleeding edge and sold at a $300 loss.