48V isn't what enables Ethernet, it does however make it more useful. 48V enables high power things to be scattered around the car - voltage loss from the front to the back of a car at 12V is an issue, and cars spend a lot of money on large wires to work around that.
High power things scattered around the car often can take advantage of Ethernet. things that cannot get a lot of power can get by on lower bandwidth data buses.
The 12V loss is something that really sunk in working on older cars. You wouldn't think a cable that read 0 on the ohmeter could still not sufficiently carry power, but it happens. Starter systems especially - My old van I remember wouldn't start once and all I had to do was clean the terminals on the battery with a wire brush.
High power things scattered around the car often can take advantage of Ethernet. things that cannot get a lot of power can get by on lower bandwidth data buses.