People who already have a lot of media they would like to be able to easily access on their TV screen, or people who are not interested in paying for content. I find Google TV more interesting since it adds an entire new layer over current television, apple TV isn't as exciting to me. I don't think people want to make a purchase decision every time they sit down to watch something.
I really like to reverse your question: why would anyone want to build their own media center now-a-days?