why are these high end monitor so expensive, at 2k I'd rather spend a bit more and get a giant OLED TV
The feature sets of this monitor and the average TV are not the same. TVs are designed primarily for viewing fixed-rate, non-interactive media.
This monitor is targeting PC games, so interactivity is key. Latency and response will be better, motion resolution will be better, allowing you to see more detail on objects in motion. Here's a basic example of motion resolution on an object moving left to right at 960 pixels per second:
And below an almost perfect example of how the object should ideally look as it's moving. This is from another monitor that supports backlight strobing.
People can try this test and more at
http://www.testufo.com/. > 60 Hz display recommended.
G-Sync mode will mean there will be no judder from Vsync, no tearing and the games will appear more smooth and consistent, whatever the frame rate.
Lastly, this display has a backlight that's divided into 384 distinct zones that can be controlled separately. This should give a pretty great HDR picture.
The display will likely be one of the first of its kind, so that'll also mean a hefty price tag.