Short answer is - Airdrop is a protocol for setting up ad-hoc wifi networks via Bluetooth LE. If it went over ethernet it wouldn't be airdrop.
Long answer:
Airdrop does discovery via bluetooth. To send a file, the two devices connect via bluetooth to negotiate the details of a new ad-hoc wifi connection, which they then create, connect to, and use for the transfer.
The discovery does not rely on a shared network, and once connected, the devices never 'discuss' whether they already share a network, so it's WIFI or nothing.
This gives some security guarantees that wouldn't exist otherwise. EG: Airdrop won't go through Starbucks' WIFI, even if both the sender & receiver are connected to it. Honestly I'm not sure how important those guarantees are -- but they're there.
I don't think there's a fundamental barrier stopping Apple from:
1. Having devices advertise an 'airdrop-box' through Bonjour as well as though bluetooth LE,
2. Including these entires in the Airdrop UI
3. De-duping entries where it figures out that a network device is the same as a bluetooth device.
4. Running same-network transfers through SMB rather than using [what they currently call] Airdrop to negotiate an ad-hoc WIFI network.
I believe the name for this ad-hoc network is Apple Wireless Direct Link (AWDL), though I think that the name also refers to the mechanism that allows one WiFi radio to be involved in doing AWDL and also not drop the connection to the WAP hosting the WLAN.
Long answer:
Airdrop does discovery via bluetooth. To send a file, the two devices connect via bluetooth to negotiate the details of a new ad-hoc wifi connection, which they then create, connect to, and use for the transfer.
The discovery does not rely on a shared network, and once connected, the devices never 'discuss' whether they already share a network, so it's WIFI or nothing.
This gives some security guarantees that wouldn't exist otherwise. EG: Airdrop won't go through Starbucks' WIFI, even if both the sender & receiver are connected to it. Honestly I'm not sure how important those guarantees are -- but they're there.
I don't think there's a fundamental barrier stopping Apple from:
1. Having devices advertise an 'airdrop-box' through Bonjour as well as though bluetooth LE,
2. Including these entires in the Airdrop UI
3. De-duping entries where it figures out that a network device is the same as a bluetooth device.
4. Running same-network transfers through SMB rather than using [what they currently call] Airdrop to negotiate an ad-hoc WIFI network.
...but they haven't done so.