Youtube can do this becuase they have the whole video before they transcode it, and by then they know its average loudness. TV sets receive the content as they play it, so they can only do this if they play the content with some delay, since they otherwise can't know in advance what the average loudness is going to be. They can of cause cap it at some maximal loudness, but that would also cap the loudness of the loudest section of movies as well, and by that reducing the dynamic range.