usb 3 is 5Gbps, thunderbolt is 10Gbps, thunderbolt 3 is 40Gbps. i don't think it makes sense to put ethernet ports on anything smaller than a rack blade.
They can also only go 5m, 3m and 3m, respectively, before interfacing to a computer (which then uses ethernet to go somewhere else anyway).
Ethernet makes sense if you consider that images are going straight from the camera to the control room (which is hundreds of feet away, and possibly parked outside in a truck), and that there's likely already network infrastructure (switches and cabling) in place for dozens of other reasons.
If you're suggesting they went with a USB or TB ethernet adapter instead, think about the complexity of supporting the numerous available NIC chipsets on something like a camera...
I do. It's increasingly common to have a rolling NAS on film sets and being able to hook up the camera and storage with an ethernet cable is a no brainer for many productions. Time is money and video workflows have historically been a logistical nightmare. Generally when you're shooting narrative on a pro camera you have to stop every 20-30 minutes to swap media. I would much rather string a cable and have one less technical person and two more people in the art department for the same money. Media management is a huge bottleneck, not to mention a source of continuous low-grade anxiety.
