Tesla also went to a 48v wiring harness in some of their vehicles to allow them to power more equipment with less copper. It might be one reason why they use nonstandard connectors, so people don't attempt to hook 12v equipment to the system and also the higher voltages might require connectors rated for it.

Now they just have to take the next step and have everything in the vehicle running on PoE.

How?

When canbus is already two wires, and by definition, is a bus, so you can just keep stringing those two wires to any module you need. I know Ethernet BUSes exist, but what advantage would those have to canbus then? They're both two-wire buses.

Canbus is much slower than single-pair Ethernet. So you can get more data (more modules connected) over a single pair of wires if you switch to Ethernet.