Because most of what you look at in real life is mundane. But go find a nice sunset or a green forest, and you can appreciate it. When you see a scene in HD or 3D on the screen, there is a heavy selection bias to show you pleasant scenes that most people seldom see in real life. If it was super 4k and 3D, but it was just your same living room you see every day, it would be uninteresting. But the same camera showing a living room in a 10 million dollar house would be interesting. (And the natural views outside even more so, most likely.)
It’s a third party kernel module, which Microsoft would love to be able to block, but legally can’t. It’s technically possible to write a virus scanner that runs in user space instead of the kernel, but it’s easier to make sure everything gets scanned if it’s in the kernel.