Because it wouldn't cost you like $2000?
Assume the TV was a modest size--40''--there's $400; with a high-end GPU--the equivalent of a 7950--there's another $200+; some SSD--32GB?--there's $100+; a tablet there's $300; and a keyboard, mouse, and controller adds another $100. There's $1100 worth of electronics, but you know they'll charge a premium for having such an all in one system, so assume $1200 for that 40'' TV with gaming capabilities, and probably $300 for a Tablet, and $100 for inputs. That's a conservative guess at $1600 for what you're talking about.
You're going to tell me you'd rather have all that shit than just buying and assembling a PC for about $600-800 and getting a bigger or better TV without the gaming capabilities, and the input devices (KB\M\Controller) which would run you like $1200 total? Or a gaming console for $300 and a (better) TV for $600-700--total of $900-1000?
People need to realize, Tablets are just the new in tech item. They won't ever completely replace Game Consoles or Desktop PC's, because they will never be as versatile. Their mobility and low profile forces them to make sacrifices when it comes to power, performance, control, and responsiveness. I hate playing most games on my phone or a tablet, because you always lack the precision you get with physical buttons, and I know most people feel the same way. Yes, games like Angry Birds work great on a tablet, but try playing something like Gran Turismo, or Gears of War, or Zelda on a tablet, and then tell me it's just as good.
Also, as for the Tablet mentioned, that was a Tablet with a dual-core i5 with HD4000 iGPU, 2GB RAM, and a 64GB SSD. The exact one they reviewed cost $850-900. Not exactly a cheap investment.