Why doesn't the background resolution change as you change resolutions?

[worldofneil]

How about each person look at the screenshots I posted. one is the 4k and the other is the 2k and you can clearly see the same fov for both. Monitor aspect ratio was 16:9

They're both the same aspect ratio so you get exactly the same field of field. The resolution is irrelevant, the game calculates the field of view based on the ratio.

according to asus that should not matter

That Asus advert you linked simply shows how many more extra pixels there are compared to a regular screen. Whether a game actually supports showing more in that extra screen space is another matter. As you've seen with FF XIV, the game is designed around aspect ratios as to how much you see, not specific resolutions.

Edit: Also just saw from your link about the monitors, there was also some text above that picture you posted, that said:

This gives you the benefits of greater image detail and up to 77% more onscreen desktop space than standard Full HD (1920 x 1080) displays.

More space, nothing about how games will handle it.