Because it can't be implemented without low level hardware access. But also, it seems like it's a part of GMS, not of the OS itself.

Low level hardware access for opening a file and a network port? Those are some of the first lessons in any programming tutorial. If they aren't available, what is the OS even doing?

Also, for all intents and purposes, GMS is part of the Android OS, but Google had to branch it off, to keep it closed source.

AirDrop doesn't open a network port, it creates a WiFi Aware advertisement and a WiFi Direct connection. However I thought this also should not need OS-level changes, just android.permission.NEARBY_WIFI_DEVICES permission.

AirDrop is based on AWDL, which is a proprietary protocol that requires low-level access to the Wi-Fi radio to implement. Apple told the EU that they have long-term plans to migrate from AWDL to the standard Wi-Fi Aware, but have not yet done so (and the EU did not require them to, only that they bring their Wi-Fi Aware support for third-party apps to feature parity).