I think at the time the 5k Mac screens arrived only DisplayPort 1.2 was available, and probably some of the chips they use still only support that. But indeed - since time moved on and DisplayPort can now do 8k/60Hz it would be nice to see more screens going beyond 4K and supporting it.
the proof is that video cards and monitors and TVs either simply don't support that bandwidth, or they're so expensive that they're essentially hand made.
That's how you know, you look at the market and see what's available, and what it costs. People would buy the heck out of this stuff if it were possible to make it work with common, sloppy, reusable connectors like HDMI or DisplayPort. The fact of the matter is that it isn't possible to make this stuff with removable connectors, yet.
That's why you see very high DPI displays in applications where there isn't any need for high cycle-count connectors, or only single-use connectors well before you see the same displays with high-cycle count connectors.