Because the U.K. is not self-sufficient. Actually, I'm fairly certain that no country in today's world is self-sufficient. That means that for people to get by,

Because the U.K. is not self-sufficient. Actually, I'm fairly certain that no country in today's world is self-sufficient. That means that for people to get by, day-to-day, the U.K. will need to import "stuff". If they were to simply up-and-leave the EU, chances are that the EU would retaliate by: * closing borders with the U.K. * embargoing or imposing severe tariffs on U.K. exports * raise prices on U.K. imports to cover the monies not paid by the U.K. So the U.K.'s choices are to: * pay, in effect, what they owe (in the forms of tariffs and penalties) with zero bargaining position * find enough other countries to trade with (who are willing to also thumb their nose at the EU) to make up for the goods and services they had been getting from the EU * negotiate a clean exit