To understand why, let's use a football parallel. Brexit Day is the final whistle. At that point, the result stands. The ref may face investigations.
Brexit's the same. Once we pass 11pm on 29 March, we're out of the EU with all that implies. There's no rewind button on international treaties.
But it won't stop Brexit. Not in the time we currently have left.
And that blunts its utility as an "immediate" argument to deploy against Brexit.
But right now, in the *69* days we have left, we must seek salvation elsewhere.