There's the LG Ultrafine 5K. Apple even sells it. I'm pretty sure it's literally the same panel as the iMac.
The iMac except for the Pro aren't easily VESA adaptable after purchase. Like you can't remove the original stand. Really it should be at the very least height adjustable at this point. But it's pretty sad when a $200 monitor will have better ergonomics than a $2000 all in one.
The desktop monitor market is mostly driven by PC gamers and businesses who don't want the highest resolutions for different reasons.