Product differentiation. If you run the head scale server, I’m not certain the funnel service will work. And there’s probably other stuff that is not planned to work with the FOSS coordinator server. And of course, one of the big things is: they don’t have to support anything with headscale, even if it’s sort of their software. Use the official stuff, and you get support. Pay, and you get better support.
Honestly that makes it even weirder.
Usually companies will just not acknowledge that an open alternative that plugs into their existing product exists, and if they do it's for enforcement/diverting purposes ("Don't use this, please stick to the official stuff")
If you're at a point where you acknowledge, accept, and even help the unofficial FOSS alternative, why not make your official stuff the same way?