Because AirDrop doesn't use your local network at all, it creates a direct peer-to-peer wifi connection between the two Apple devices. It has no way to do that via an ethernet cable.

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.

...but they haven't done so.

I used to transfer large media files directly between Mac and Windows using a crossover cable. Peer-to-peer without a LAN. The good ole days.

As someone said in a nested reply: you can. You don't need to be connected to anything via Wifi, but it needs to be on.

This isn't just theory: I use my Mac mini in exactly this configuration. Wired Gbit Ethernet, but with Wifi left on but connected to no network for airdrop/handoff/etc stuff.

this is an anti argument to me. you should be able to, over bluetooth. or maybe mdns, in some slightly better fantasy world. but this question distracts, picks foibled with airdrop, when a far greater scope question of why no one anywhere has low-intermediation direct device-to-device discovery. I don't care that Apple's implementation isnt perfect, doesn't work for xyz use case. no one else has any airdrop like basis at all. and there are no protocols or standards out there that begin to address this fault. everyone else is on square 0. terrible.