Because exit might be a variable you use to determine if you should exit. exit() is a function that actually does the exiting.

It’s the difference between pointing at a jogger and saying “run” and actually running after them.

This is the code (Github link):

class Quitter(object):
    def __init__(self, name, eof):
        self.name = name
        self.eof = eof
    def __repr__(self):
        return 'Use %s() or %s to exit' % (self.name, self.eof)
    def __call__(self, code=None):
        # Shells like IDLE catch the SystemExit, but listen when their
        # stdin wrapper is closed.
        try:
            sys.stdin.close()
        except:
            pass
        raise SystemExit(code)

What happens is that the python repl calls __repr__ automatically on each variable/statement that you type into the repl (except assignments e.g. x = 1). But this basically only happens in the repl. So “executing” only exit wouldn’t work in a python script as it is not calling __repr__ automatically, so better you learn how to do it right than using just exit in your python scripts and scratching your head why it works in the repl but not in your code.