1) Long distance communications require much more powerful transmitters/receivers, which also makes it hard to design for high data rate. 10 Gbps ethernet is only just getting popular for consumer electronics, while 10 Gbps USB and video have been around for a very long time. It's not just market forces, it's inherent in the physics of the interface IC design. It also impacts power consumption; a 10 Gbps ethernet PHY takes a lot more power than a USB3.1 hub (by like, 10x per port).
2) Ethernet is galvanically isolated. This takes up a huge amount of space, both volume and PCB area, and makes it much more difficult to provide power. Galvanic isolation is important for robustness across variable electrical environments, but isn't that important for devices in the same room, and especially not on the same AC circuit.
3) The connector is optimized for cost and easy installation in the field, but it's not particularly durable or electrically robust (locking tab is delicate, can't be overmolded easily, not electrically shielded most of the time, huge transmission line stubs, etc.).
I can definitely envision a world where everything used maybe two standards, perhaps Ethernet for long distance and Thunderbolt for short, but there are lots of valid reasons for differentiation. There will always be tradeoffs. In the imaginary two-standard world, you wouldn't want to pay for a 20 Gbps Thunderbolt controller in your mouse.
Not for high speed data (USB3.1 ++). The electrical length of the unused pins causes reflections in the signal. It's a consequence of the finite speed of light.
I _really_ like KiCAD, it's my preference for anything simple because I absolutely love the workflow compared to Allegro or Altium (especially Altium). I actually really enjoy the library management as well.
That said, anything using an HDI stackup or a reasonable number of differential or matched traces, KiCAD becomes a struggle. Even simple things like a single pair can be pretty tricky if you have space constraints. These features are pretty fundamental for modern PCB design.
It seems like KiCAD v6 will be making a lot of progress to manage more complex stackups and impedance control, so I'm very excited to try it.