Why can't I Airdrop from my phone to my MacMini thats connected via ethernet cable. They are on the same network, and verified with other tools, but AirDrop has to be done over wifi for what reason?
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.
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.