Because it’s draining, destracting and destablising.
Nope. We don’t want the UK involved in framing long-term strategy that they may not stick around to implement. It’s hard to plan our budgets and programmes on a 7-year cycle if one of our major contributors (and one of our major recipients) is not likely to be around for that 7 years, but we have no idea when they’ll bail out.
The Lisbon Treaty provides for a two-year (but flexible) period for a reason; prolonged uncertainty is damaging.
The irony is that the UK did debate the issue for years, and had they chosen to they could have debated it for more years, before invoking Art 50. It’s just it was a really poor debate, and didn’t produce any clear position on the objectives, strategy or even the the point of Brexit, and was largely conducted in a reality-free environment in which wishful and magical thinking went unchallenged. And, frankly, there’s no reason to think that had they debated for longer than they did, they would have debated any differently. It seems to be only the pressure of a deadline that has introduced any realism into the UK’s debate.
We don’t want to break up, but we accept that the UK does. We;re not going to be the clingy ex; we have other stuff to do. And by now we’re getting a bit impatient to move on and do it. Our objective at this point, and for quite some time past, has not been to avoid the breakup but to complete it with minimal harm to ourselves.