Hinges are no. 1 wear sensitive component in flip phones and laptops. They must be properly engineered. Even some cheap flip phones had slip rings for display connection 15 years ago, why Apple can't do the same?
As much as I want to agree with you, those old phones had low resolution, low refresh rate screens. You need a much higher quality connection to push UHD video down, and I doubt you can get a slip-ring to reliably carry signals of that high a frequency. The capacitance and signal reflection issues would be tricky to work around.
