I think the thing that makes an OSS implementation more difficult than iOS/macOS is the friction involved.

Say you've got an android phone, windows PC, and a linux box, and you want to be able to quickly drop files from each one. unless we get some kind of cooperation across all three platforms at the OS level, you'd at minimum need to install some kind of client into each system - when the nicest feature of airdrop is that it's baked into all of Apple's OSs, in my opinion. even if it worked exactly the same way, but had to be installed, I think it would see less use - and there's no real way for a single OSS project to do that across multiple OS platforms, to my knowledge

The physical layer part really isn't complicated, and most Wi-Fi chipsets have supported something like it for over a decade now.

What's tricky is to specify and get everybody to implement the layers on top of it, including device discovery (frequently offloaded to Bluetooth for efficiency reasons), user identification (Apple runs a PKI for this) etc.

Not an expert on mobile development but I doubt an android app has the low-level access needed to the wifi stack to do this.