why is there no 1440p game clip support

Probably just one of those features they missed. I would suggest posting this on the xboxideas.uservoice.com website. The more votes it gets, the more chance it will be implemented in a future update.

I have checked the manual, it says it supports 1440p 120hz over hdmi the only thing it supports over display port is 1440p 165hz. So I will ask again why won't it let me set it manually hdmi then choose 1440p 120hz, but the option is available with auto detect. The monitor also supports VRR and free sync over hdmi.

That monitor supports 120hz ONLY at 1080p, and 60hz at 1440p according to the companies website. That would be why you cant select 120hz at 1440p

"With our monitors, you can run 120Hz at 1080p resolution. Or you can rock a pixel-perfect 1440p resolution at a 60Hz refresh rate. Even knowing this, you’re now able to enjoy buttery smooth gameplay
on one of the most popular game consoles of all time — thanks to FreeSync support"

From viotek website

Hi,

It's not possible on that display. The big reason why is that it isn't a 1440P display, at least it isn't a 16:9 aspect 1440P display, it's a 21:9 Ultra-wide monitor. Xbox only supports 1440P 16:9 aspect ratio displays. These monitors have a resolution of 2560 x 1440, yours has a resolution of 3440 x 1440.

The Asus ROG PG348Q is designed more for PC gaming not console gaming. Unless Microsoft provide support for these types of monitors in the future I'm afraid you're stuck with a maximum resolution of 1080P.

So what options do I have? Only 1080p

Hi,

A resolution of 1080P is 1920x1080, while 1440P is 2560x1440. Since your display doesn't support 1440P, the highest resolution would be 1080P.

To get 1440P would require a PC gaming monitor that supports that resolution.