Because it's a hobby, just like the Apple TV.
When people buy a new TV, it's because their current one died on them, or they don't want to carry it with them when they move or something. There are people who are into buying massive TVs every year, but very, very, very few. It's too small of a niche market that wouldn't make the production cost worth it.
You're not sitting a chair away from the TV either, so Retina isn't going to make a difference. The panels on the regular MacBook Pro (with an update) make more sense.