Because Apple does not NEED to make every single necessary object in the universe. LG is making them, they are 5k, they have wide color gamut and 3x USB-C out, and they are in the Apple Store. That's good enough. This enables Apple to focus on what they are good at.
If we stretch this approach, Apple will become good at making the iPhone and nothing else.
The display was the soul of the desktop Mac. It's easier to perceive your Macbook as another PC when your window into it is another mostly generic-looking LG display, even if the panel in the Apple branded display was manufactured by LG.
Perhaps they want to focus on the iMac again and don't want the MBP + display combo to cannibalize it. But again, with extreme focus soon they'll be abandoning the iMac for good.
(I still love macOS and I enjoy how the new MBP looks)