Earlier quoted context omitted.
Because this way the screen can turn on and off dynamically. Also, it doesn't tie you to having to have a TV in a location you want one.
HDMI-CEC allows similar on most modern TVs. As for not being tied to where you have a TV, for $200 you can buy a 40"+ 1080p TV, for $100 you can have a really nice IPS 22" 1080p monitor. Probably better than this tiny thing.
My TV takes a good 10 seconds to start producing audio after the google chromecast 'on' command. Putting the screen in themselves gives them control of that experience.