I don't think that's correct. It's not "all video" - you can easily encode video without chroma subsampling - and it's not because this is how analog TV worked, but rather for the same reason why analog TV worked this way, which is the fact that it lets you encode significantly less data with barely noticeable quality loss. JPEGs do the same thing.

It's a very crude method, with modern codecs I would be very surprised if you didn't get a better image just encoding the chroma at a lower bitrate.