Because none of them actually match the capabilities of AirDrop, since they essentially require controlling the full stack (UI, low-level networking including Bluetooth for discoverability, Wi-Fi peer to peer connections without dropping any existing infrastructure connection etc.)

Many have tried, I don't think anyone has succeeded.

Supposedly the EU interoperability mandate will make this possible going forward, though? (The tricky part is usually not getting your device to speak some protocol, but to get Apple devices to actually respond to your attempts.)