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.