Additional ports would complicate the user experience. The Studio Display has no buttons on it, but if you added additional inputs, you would also need to add a button for input switching at least. And potentially other buttons for brightness and volume settings.
It may not sound like a big deal, but I have an LG monitor that uses a remote for input switching and volume controls, and a BenQ monitor that uses buttons, and both provide a noticeably jankier experience.
The Studio Display provides a very clean user experience when paired with a Mac. You plug it in, it turns on, and all other functions (volume, brightness, colors, camera, etc) are controlled via MacOS. Personally I'm happy for Apple to optimize for that experience, at the cost of not working with non-Apple devices.
