The issue, specifically, is that the display ribbon to connect it to the board needs to go somewhere [0]. For years, it was the bottom. So when it comes to removing a chin, you need to find a place for that ribbon. Apple achieved this by folding the screen at the bottom, so the screen isn't technically ending at the bottom. This is why it's easier to stretch out screens to the sides and the top.

[0] https://www.theverge.com/2018/3/28/17152840/oneplus-6-notch-...

You might want to check the reason for the limitation before making such comments. A display can be stretched to the three edges of the phone —- it is the fourth edge that is the problem — as Xiaomi demonstrated with Mi Mix 2. At least some space is needed to house the display driver, camera, sensors etc.