Because there are issues that need to be worked out. All the trade arrangements they hold within the EU could be overturned if they just bolted without negotiation, for example. What happens to British firms that have holdings in EU regions? What about the reverse, EU banks or financial institutions with major centers in London? British expats living in, say, Spain, what happens to them? Et cetera.
They need to work this out and the EU is going to go out of their way to make it extra painful. But if they simply walk out it would be more so.
Jeez, I’m not an MBA, but I would hope any sizeable contracts would include language on what would take place in the event of a bug out. I’m just really concerned that Remainers are going to use that 2 years to throw a spanner in the works.