More's the point is actually 'why do you think we will be able to?' The spurned wife in an acrimonious divorce, left with the young toddler children and truculent teenager to bring up, is not always that accommodating of the husband who breaks from the family to go seek new love, fame and fortune free of the straightjacket of the marriage.
Absolutely STAY/IN
The UK runs a massive trade deficit with the EU. If the EU imposes import restrictions or tariffs and the UK reciprocates, the UK generates money to subsidise trade - a UK win. The EU are unlikely to impose restrictions as any reciprocation will hurt them more.
I'd expect the Article 50 negotiations would involve a large 'penalty' for leaving, a divorce settlement using your analogy.