For many the TV you buy is a complicated matter, and for many others there's going to be price issues. An Apple built TV would just be super expensive and Apple would have to quit making the AppleTV so people would be forced to buy their Apple built TV.
That would never make any sense. The AppleTV is a gateway to iTunes movies and rentals, and all those people who purchased content on them would go ballistic if they were suddenly told they have to buy a Apple built TV if they wanted to continue to watch their purchased content on a TV.
Apple can make a difference in DVR's and a over the top tv service, and that should be their focus in this space.