Great comments – just to respond to a few:
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Thunderbolt is closely related to PCIe - a relatively direct communication with the CPU, whereas Ethernet is a 'bolt-on' for network communications and requires translation to 'ip'.
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AVB operates at Layer 2 so does not go through the IP stack. The default latency (presentation time offset) specified by the AVB spec is 2ms for Class A traffic but hardware implementations such as RME’s can tune this down as low as 0.3ms. This does require cooperating devices for low latency in both directions as the offset is set at the output (”talker” in AVB-speak). It would be difficult for a non-realtime software implementation such as the CoreAudio one to achieve these latencies though and you are correct that PCIe and even USB interfaces are able to offer lower latencies.
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Thunderbolt is fast (40Gb/s) and Ethernet is slow (10Gb/s if you're lucky, more usually 1Gb/s).
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Audio bandwidth is relatively tiny, 1Gb/s is enough for many hundreds of audio channels (particularly in the case of AVB where the switches cooperate to reserve bandwidth).
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Thunderbolt is a point-to-point communication medium, intended for the CPU to talk directly to peripherals whereas Ethernet is a many-to-many communication protocol with lots of things built in to deal with collisions and delays in devices speaking to each other. These overheads create inefficiencies.
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Not really an issue with modern Ethernet switches because each port is its own collision domain.