Chromecast is typically powered by the USB port on the TV, not the wall. I'm not going to buy this if I need to plug it into the wall.
Chromecast is powered by USB, whether that comes from the TV or the wall depends on whether the TV has a free USB port (which requires having a USB port to start with.) I suspect lots of Chromecasts are used with older, pre-USB TVs (the popularity of TV USB ports largely coincides with Smart TVs, and Chromecast adds more to a TV that isn't already a Smart TV.)