That’s because a lot of broadcasters are either not transmitting 4k or encoding their streams in such horribly low bitrates that you might as well be watching through glasses made from empty beer bottles.
Contrast Apple TV with YouTube; or Crunchyroll vs Youtube. Then step it up to BD. There is such a huge difference between 4K “fast”, 4K “main”, and the 4K “high” used on BDs.