Why is Main method private?

Yes. You can mark the main() method as public, private or protected. If you want to initiate the entry point by any external program then you might need to make it public so it is accessible. You can mark it as private if you know there is no external usage for the application then it is better to make it private so no external application access it.

public class MainMethod
{
    private  static void Main(string[] args)
    {
        Console.WriteLine("Hello World !!");
    }
}

The entry point of a program is marked with the .entrypoint IL directive. It does not matter if the method or the class is public or not, all that matters is this directive.

The Main method shouldn't need to be called by anyone.

It is actually marked as the entry point for execution in the EXE itself, and therefore has no outside callers by default.

If you WANT, you can open it up to be called by marking public, e.g. if you are turning a console app into an API.