I think the devs commented before that the way this UI is designed, it's all a bunch of bitmaps overlaid over the viewport. So in order to add more scaling options, they have to sit there and create new UI elements for every single tiny bit of the UI. All 10,000+ elements. One by one. It's a horribly inefficient design but hindsight is always 20/20. From the original standpoint of only needing to support 720p (PS3) and 1080p (PS4, PC) resolutions this UI design probably made sense during the crazy 2 years of crunch time dev cycle to remake 1.0 into ARR.

I personally wouldn't mind if they just took the largest bitmaps (currently 140%) and just let you scale them up more. I don't care if this makes the UI elements look blocky, I'll be too busy looking at the game being rendered in 4K anyways.