There are a lot of native size requirements that third party monitors don't meet as well. If you want 5K, which is the ideal for 2x 2560x1440 at 27", you're in a very different market. A 4k display running at 1x is easy to find, but a good 5K is a very different story.

This is why Apple sells the LG 5K display and why their iMacs have 5K displays when possible. Third party displays do work, but when you want to get into crazy town resolutions it's much harder to find them with high quality.