You might ask the same question about Trump. Surely someone in charge - someone with authority - should have seen the trainwreck a mile off, and done something about it?
The shocking truth is that there's no one with both common sense and overriding authority that prevents catastrophic wrong turns. This was absolutely an expected outcome of Brexit, by anyone with a clue. The trouble is that half the general population doesn't have a clue. Why did the government ask them? Because the party in charge at the time had a splinter faction led by some very radical, very loud voices, and it was beginning to cost them votes. They hoped to put the issue to bed by calling a referendum. Having done so, it was political suicide not to follow through even though it wasn't technically "binding".
Largely it's a matter of various people acting in their own selfish best interest at the expense of the interests of the country, and convincing a bunch of useful idiots to follow along.