My guess is that it's difficult to interface with the system's Bluetooth and WiFi sufficiently without a native app on any modern platform (iOS, Android, Mac, Windows, Linux) enough to create and advertise that kind of ad hoc network, without a native app on the device (perhaps even with system permissions).

Since Apple won't implement any third party one, and theirs is natively integrated with their platform, half the ecosystem won't implement or adopt any FOSS alternative.

Since such an alternative won't be pre loaded on handsets (and the Android ecosystem is complex without one single vendor producing firmware everyone ships), the rival would need to be installed manually by users before use.

Not impossible - WhatsApp and other apps have (in some markets) gained near-ubiquity without being built-in, but I think the native app barrier here will always be a hurdle. And Apple presumably knows and strategizes that an alternative won't gain adoption if their half of the ecosystem won't adopt it, therefore holding back the wider market and keeping airdrop functionality as a USP.

If that were the case someone would at least have made a version for Linux devices since you can have full access to them

There are plenty of competing niche apps doing pretty much that. xkcd.com/927 applies here.

pretty much = "they don't do what you asked"

There is KDEConnect, which has apps for all major platforms (iOS, android, macOS, Windows and of course Linux) and some more. I even used between Apple devices when AirDrop did not work for some reason.

https://kdeconnect.kde.org/

KDEConnect does not implement what I've said

It doesn't?

Does it create a seamless p2p wifi connection between any pair of devices it's installed on?

Kde connect requires two devices already connected to the same local wireless network.