It might be necessary to create a legal basis, but it's just a matter of doing it. If the owners don't like it they can dissolve it.
Tell me you don't know anything about the law without telling me
That's not the sole reason. They (should) also enforce a fair even playing field by preventing market manipulation (e.g. how Elon was tweeting about stock prices) and a few other things to "facilitate efficient markets and the formation of capital".
> Why should SEC rules apply?
Because private companies still fall under the jurisdiction of the SEC? See e.g. Theranos.
> On what legal basis can you force a private company to divulge its financial details?
On the to-be-created legal basis that aims to prevent bubble formation and the resulting fallout to the wider society?
> Would you be happy if you, as an individual, have to divulge your account statements if your own net wealth reaches one million?
Sure, why not? It's not a totally unheard of idea. In Norway everyones salary can be looked up.