I don't care why. They do nothing to circumvent this so they are not a reliable solution for those who have network participants using the restricted platforms.

There could be a million reasons, but not a technical one — "headscale client", for example, could exist in current hostile app stores, but there isn't one.

Your arbitrary, loosely-detailed complaints would apply to literally everyone, every app.

It's on f-droid, it's open source, you're being ridiculous. I'm not even sure you understand what you're asking for. The official, open source Tailscale client explicitly supports headscale servers.