CGP grey did a video on it: https://www.youtube.com/watch?v=iHzzSao6ypE
Basically one car breaking too much will make the following brake even more and so on until one stops and there’s a jam. There is no clear reason like a road blockage or an accident, just compounding slow down.
Not only the content doesn’t exist yet, it’s just not practical. Even now 4k broadcasting is rare and 4k streaming is now a premium (and not always with a good bitstream, which matters a lot more) when once was offered as a cost-free future, imagine 8k that would roughly quadruple the amount of data required to transmit it (and transmit speee is not linear, 4x the speed would probably be at least 8x the cost).
And I seriously think noone except the nerdiest of nerds would notice a difference between 4k and 8k.