Because Apple already sells a $99 device that makes little profit. They want the "value added" on a TV set -- not compete with razor thin margins on a flat panel.
The Apple TV will have best of class quality, and it will accomplish things that a lot of technology challenged home users are going to think Apple invented -- because they've never been able to do them before. A lot of critics say that Apple fanboiys think Apple invented everything -- when that's really the case with NEW USERS. Sure, those Android phones could do X, Y and Z if you turned the right screw -- but the CONSUMERS don't know that until it's accessible.
Apple is creating the Full Widget because they they can control the experience and charge higher margins. What else is new?
The gen2 aTV had a BOM estimate of $64. While margins are thinner than Apple's very high average but we're nowhere near in Fire territory.
I have no idea what folks think an Apple branded HDTV can do that the aTV + any other HDTV can't.