Automotive ethernet over copper physical layer is completely different than commercial/data center ethernet. It is defined in 802.3ch for multi gigabit (2.5, 5 and 10 Gbps). It uses single shielded twisted pair cables up to 15m long. Cables and connectors are automotive grade.

The power over data line (PoDL, automotive Ethernet equivalent of PoE) is defined by a separate IEEE protocol and its critical specifications like EM emissions tests, ESD tests etc are supplemented by documents created by a consortium of car, electrinics, comnector, cablr and semiconductor producors called OPEN Alliance: https://opensig.org/ . There are parts available for PoDL. The voltage levels from 6V to 60V with 6V increments are supported.

Source: I design both data center and automotive Ethernet chips.

Historically and increasingly, automotive grade electronics are a separate genre to general electronics for reasons of safety.

It's less of a safety but more of a production and environmental constraints issue. Cat5 cables are too heavy for cars. Their connectors aren't made for vibration. They have a lot of emissions which is a problem for the mission critical parts (Ethernet Phy often isn't mission critical). They are expensive, which is actually the biggest reason they aren't good for automotive.

Automotive reliability is only an issue for your ABS sensor, airbag sensor etc. but these are a minority compared to what's in modern cars these days. Real driver is cost, compactness (cost) and harsh environment (temperature and vobrations) and EM emissions. It mainly adds to qualification time, but actual semiconductor design cycle isn't that long. That being said, data center stuff is also notoriously slow to qual.