This question was posed in a below the line thread in the Guardian.  My reply:

Because:

(1) MPs wrongly, and without permission from the electorate, abrogated their responsibility to be the representatives of their constituents by voting to have a referendum.

(2) The wrong question was asked in the referendum (though to be fair it is almost impossible to formulate a good question—just one of the many reasons referendums aren’t democratic); and the context in which the referendum was happening—specifically that it would be an advisory result—was (deliberately?) clouded.

(3) Campaigners on both sides of the referendum—but mostly leavers and, most egregiously, leavers—lied and cheated and have been allowed to get away with it.

(4) Those who aspire to be leaders, like Corbyn and May, and their various acolytes have behaved with staggering incompetence and stupidity ever since the result was announced.

But, and I think this is your real question, how could this possibly all have happened?

The answer is simple and was encapsulated by Heidegger some time ago *1:

The most thought-provoking thing in our thought-provoking time is that we are still not thinking.

Russell had already observed *2.

Most people would die sooner than think—in fact, they do so.

And George Carlin explains why these two statements are true *3:

Think about how stupid the average person is, and then realize that half of ’em are stupider than that.

The fact is that Brexit, or reducing global warming—let alone literally dozens of ‘lesser’ issues, such as developing a useful prison system—are challenges which are simply beyond the intellectual and moral skills of the people we vote into power to deal with them. And that is partly because anybody competent and intelligent enough to deal with these issues constructively is intelligent enough to resist any thought of going into politics as currently organised. So we end up with a huge illustration of the Dunning-Kruger effect *4:

…a cognitive bias in which people of low ability have illusory superiority and mistakenly assess their cognitive ability as greater than it is. The cognitive bias of illusory superiority comes from the inability of low-ability people to recognize their lack of ability.

This situation is not capable of resolution.

*1  Martin Heidegger, What is called thinking? [Was heisst Denken?] (1951–1952)
*2  Bertrand Russell, The ABC of relativity (1925)
*3  George Carlin, Doin’ it again
*4  Wikipedia, https://en.wikipedia.org/wiki/Dunning–Kruger_effect

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