We do! 2018 iPad Pro has 4k output.
Of course this feature currently depends on each individual app to implement secondary display output at 4k. I haven't used it so I'm not even sure what happens when you switch apps... Does the external display go blank? switch to mirrored display?
So yeah, there's room for improvement. But how would this look? iOS would have to enable secondary display system-wide (not mirrored) and then all apps would need to be re-coded with a new 16:9 display format option, and new display formats for split-view, etc. It seems like it would be a mess, a lot of work, and I'm not sure if most developers would see the value of it, considering how must users will never attach an external display.
All that to say, I agree with you but I am skeptical that it will happen. I think our best hope is for app developers to get more innovative in how they implement their own 4k secondary display output. LumaFusion for video. Adobe Photoshop (supposedly coming in 2019). Any remote desktop app. These types of apps could be built to make great use of external display, no iOS system changes needed.