Not least because we may have triggered Article 50, but we haven't left the EU yet.
According to the guy who actually drafted Article 50, we can reverse the process, but then, what would he know, right?
Brexit is reversible even after date is set, says author of article 50
The former diplomat who drafted article 50 says the UK could opt to reverse Brexit up to the moment we leave, even if a date for the country’s departure from European Union were added to the withdrawal bill, as Theresa May plans.
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Lord Kerr, a former UK ambassador to the European Union, said Brexiters in May’s cabinet were suggesting Brexit was irreversible and thereby misleading the public.
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Speaking on BBC Radio 4’s Today programme Kerr, now a crossbench peer, said the UK could still opt to stay in the EU. “At any stage we can change our minds if we want to, and if we did we know that our partners would actually be very pleased indeed.”
He added: “The Brexiters create the impression that is because of the way article 50 is written that having sent in a letter on 29 March 2017 we must leave automatically on 29 March 2019 at the latest. That is not true. It is misleading to suggest that a decision that we are taking autonomously in this country about the timing of our departure, we are required to take by a provision of EU treaty law.”
https://www.theguardian.com/politics/2017/nov/10/brexit-date-is-not-irreversible-says-man-who-wrote-article-50-lord-kerr
There's much legal debate (usually focusing on whether the UK could reverse course unilaterally), and if it did happen, it would most likely require the consent of the other EU members (and no doubt a punitive quid pro quo to stop the UK any other member using such a declaration as a bargaining tool in future), and since there's no precedent, possibly decisions by European courts on the matter. Here's four legal opinions by European law experts:
Stefan Enchelmaier, professor of European and Comparative Law at Oxford University, argues that the essential requirement of withdrawing from the EU is the simple desire to leave. "If you look at Article 50, first you have the decision to withdraw, then you have the intention to withdraw," he says. "No withdrawal without notification, but, the other way round, no withdrawal without the intention to withdraw." If public opinion reversed and the UK wanted to remain, therefore, this would legally be possible.
Kenneth Armstrong, professor of European Law at Cambridge University, says this is the "dominant legal opinion" on the subject and the authors of the Article are in agreement.
However, Article 50 is not very well written. There is no explicit provision for this. Equally there is no precedent for withdrawing from the European Union, so there is little to go on. Jan Komarek, a lecturer at the London School of Economics' European Institute and Department of Law, says that, as Article 50 is "silent" on whether a withdrawing member state could change its mind during the negotiation period, lawyers would have to look for other examples in international law. Under the Vienna Convention, for example, a "fundamental change of circumstances" is grounds for withdrawing from an international agreement, provided "the existence of those circumstances constituted an essential basis of the consent of the parties to be bound by the treaty".
In other words, if the UK no longer consents to being part of the EU, it may leave the EU. If that situation changes and the UK decides to remain, it may remain. "When a state which is party to an international agreement wants to revoke the agreement and then wants to revoke the revocation of the agreement, that is possible according to the Vienna Convention," Komarek says.
http://www.politics.co.uk/comment-analysis/2016/12/12/can-we-withdraw-article-50-once-we-trigger-it-probably-yesThis is also allegedly the finding of a controversial legal opinion the government's trying hard to keep secret because democracy and the Will of the People:
It’s official: Article 50 can be withdrawn
Jessica Simor, of Matrix Chambers which was co-founded by Cherie Blair, has said she believes the document that signalled the withdrawal process from the European Union can be reversed.
She also claims to have been told by two sources that Theresa May has been advised Britain is not legally bound to quit the EU.
Writing in the Observer Ms Simor said: “Article 50 provides for the notification – not of withdrawal but of an ‘intention’ to withdraw.
“In law, an ‘intention’ is not a binding commitment; it can be changed or withdrawn. Article 50(5) is, moreover, clear that it is only after a member state has left that it has to reapply to join. Had the drafters intended that once a notification had taken place, a member state would have to request readmission.”
http://www.theneweuropean.co.uk/top-stories/it-s-official-article-50-can-be-withdrawn-1-5229073
This is also the opinion of the European Commission:
EU confirms Article 50 can be reversed — just not by Britain alone
The EU has confirmed the UK cannot unilaterally reverse Brexit – while simultaneously leaving the door open for Britain to remain part of the European Union.
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In a fact sheet released on Wednesday, the European Commission stated that “it was the decision of the United Kingdom to trigger Article 50. But once triggered, it cannot be unilaterally reversed”.
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Indeed, French president Emmanuel Macron said last month that the door was “always open” for Britain to remain in the EU.
His comments were echoed by German Finance Minister Wolfgang Schaeuble, who said that “If they wanted to change their decision, of course they would find open doors, but I think it’s not very likely”.
https://uk.news.yahoo.com/eu-confirms-article-50-can-reversed-just-not-britain-alone-212840345.html
Late last year, a Brexit Minister was forced into an embarrassing apology to the House of Lords for making a claim like yours:
Brexit minister apologises to peers for saying article 50 cannot be revoked
Lord Callanan, who was appointed a Brexit minister last month to replace Lady Anelay, who stood down for health reasons, has just apologised in the House of Lords for telling peers last week that article 50 could not be revoked.
Asked a week ago today to confirm that the supreme court, in the Gina Miller case, said article 50 was irrevocable, he said: "I can confirm that. It is also stated by the European commission that article 50, once invoked, is irrevocable unless there is political agreement on it."
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And today Callanan did apologise. In a statement to peers, he said that his statement last Monday was incorrect. That was a result of “a misunderstanding of the question”, he said.
"To reiterate, for the avoidance of any doubt, the supreme court proceeded in the Miller case on the basis that article 50 would not be be revoked, but did not rule on the legal position regarding its irrevocability. It was, and remains, the government’s policy that our notification of article 50 will not be withdrawn ... I recognise that my comments have caused confusion and I apologise to the House."
https://www.theguardian.com/politics/blog/live/2017/nov/20/brexit-bill-voters-will-go-bananas-if-uk-offers-40bn-to-eu-former-tory-minister-warns-may-ahead-of-key-meeting-politics-live?page=with:block-5a12f26160526906e8cba106#block-5a12f26160526906e8cba106
In an attempt to clarify the situation, the EU Parliament drafted a resolution:
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