Why can't modern video cameras be used for live streaming without a capture card?
I am not 100% sure but I think that there is a need for a minimal system managing the Internet Protocol in order to enable streaming. So it means there would be a need for a CPU, an OS, and a interface to setup all the related settings to the camera too.

I think the computing power needed for video compression would be possible thanks to a recent processor like the one embedded in smartphone. And of course the rate is no longer a problem for new connections. USB-C would even be able to stream raw video 4K 60fps = 12Gbps. Which is fine for be DisplayPort 1.2a available on USB-C