Not really true, tailscale clients do allow you to point to different control servers and open source implementations do exist[1] and are thriving. The clients are also open source and you can even create one yourself if you are willing to.
The thing is, what makes tailscale works really well as a "central" control server is that it makes a lot easier to connect your personal machines. You don't need to deploy your own server, or mess with networking stuff. You just download it, log-in and there you go. I myself have invited some non-tech friends to my network for playing lan games from time time and they find pretty easy to setup tailscale on their side.