Why don’t monkeys evolve into humans any more?
Because: they never did. We primates all had a common ancestor. And that species evolved into Homo sapiens and others of that genus, as well as, separately, into Pongo pygmaeus and the other great apes.
Monkeys do not become humans because the leap across the branches of the tree of life are too great. Their chance to be something different to what they are came and went a long time ago. Circumstance and geography made monkeys, monkeys and humans, humans.
Why can’t the U.K. be like Switzerland? Or Norway? Or New Zealand? Or Singapore? Or any other country that flourishes outside the European Union?
Because: each of these countries evolved into their current state, just as the U.K. evolved into ours.
I think evolution is a good metaphor for how countries develop. Thousands of random events affect their shape. Sure, there are political leaders at the helm and their decisions do matter, but so do a mess of other influences: cultural, social, economic, geographical, geological and meteorological. And where each country ends up is a product of all those factors, unique to each.
In the case of the U.K., and pertinent to our current turmoil, are some political choices that took place in recent decades. The first is the decision to enter the EEC in 1975. And the second was the Good Friday Agreement, ratified by two referendums in May 1998.1 Those treaties caused our country to evolve into the very particular thing it is today, and they blocked off other evolutionary paths at the same time.
In evolution, and in international politics, reversion doesn’t really happen. Instead, new adaptions take place which build on what is already there.
Hard Brexit seeks to undo our country’s evolution and return to an earlier time, before we were so entwined with our European neighbours — in particular, the Republic of Ireland. Change can and will happen, but slowly. To force the issue, as Hard Brexiteers recommend, would be an act of mutilation. It would be as painful as cutting off a tail, or splitting open some webbed feet. And it would be equally disadvantageous, amputating the very adaptations that suit our idiosyncratic place in history.
1. One might add that those treaties were themselves the product of seismic historical events: global war, and centuries of colonialism.