There has been some research around the APN protocol and iMessage in the past by NowSecure[0] and otherse (see links in the article). I would guess the usefullness of a reverse engineered variant is just too low given the requirement for a valid serial number for each account.

[0]: https://www.nowsecure.com/blog/2021/01/27/reverse-engineerin...

Or, people who do this sort of work don't want to encourage more people to stay in the walled garden of Apple.

I don't think that's the case. There are plenty of open source projects that open up Airdrop to devices other than Apple's.

Isn't it just a simple question of the ROI for sich a project? If you need an iPhone anyway why should you use an alternative app on another phone?

The only reasonable usecase I can identify are people writing iMessage bots/ spammers but those groups most likely wouldn't want to open-source their stuff.

For some pretty thorough reverse engineering, there's a Matrix bridge for iMessage [1] that's being used in production by Beeper [2].

[1] https://github.com/tulir/mautrix-imessage

[2] https://beeper.com/