Because the ps4 pro isn't powerful enough to run games at 4K. Aside from maybe games like minecraft or simple indie games, it cant run native 4k. It will run at 1080P, 1200P, 1440P or 1600P and then upscale to 4K (2160P).
Even the Xbox one x, which is drastically more powerful than the ps4 pro, doesn't run very many games at native 4k, it again goes with 1440P/1600P and upscales the games.
Running native 4k is incredibly taxing, and it's not something you can pack into a cheap console yet. To put it in perspective, 4k is literally 4x the pixels of 1080P. Meaning its requires exactly 4x the gpu horsepower to run games at 4k than at 1080P at the exact same settings. The original ps4 didn't even run most games at 1080P, the ps4 pro is no 4x as powerful as the original ps4. 4x more demanding than 1080P is quite a lot more than I think people realize.
If you mean why are they not making more 1440P games upscaled to 4k, the reason is still, it's not powerful enough. The xbox one x is the only console capable of playing games at 1440P. The xbox one x is much more powerful than the ps4 pro. So sony needs a new console to push 1440P gaming.