It's a Club

In an interview in a Brexit-themed issue of German weekly Der Spiegel, [Germany’s finance minister, Wolfgang Schäuble] ruled out the possibility of the UK following a Swiss or Norwegian model where it could enjoy the benefits of the single market without being an EU member. “That won’t work,” Schäuble told Der Spiegel. “It would require the country to abide by the rules of a club from which it currently wants to withdraw.

[...]

" In is in. Out is out. One has to respect the sovereignty of the British people.”

  Guardian

Sure. That's all it is. Respect for the British people.

Really, though, if the union was formed to create a single market, then it only makes sense. On the other hand, why does Switzerland get to take advantage of the single market when it isn't - never was - a club member?

“Europe will also work without Britain if necessary,” Schäuble said. “At some point, the British will realise they have taken the wrong decision. And then we will accept them back one day, if that’s what they want.”

With caveats and penalties.

That's what attracts American readers.  What's Britain and Europe have to do with America?

...but hey, do what you want...you will anyway.