Why is the main() function not defined inside the if '__main__'?

You can often see this (variation a):

def main():
   do_something()
   do_sth_else()

if __name__ == '__main__':
    main()

And I am now wondering why not this (variation b):

if __name__ == '__main__':
   do_something()
   do_sth_else()

Or at least this (variation c):

if __name__ == '__main__':
    def main():
        do_something()
        do_sth_else()

    main()

Of course the function calls inside main() might not be function calls, they just represent anything you might want to do in your main() function.

So why do people prefer variation a over the others? Is it just style/feeling or are there some real reasons? If possible, please also link sources.