The general sense is that elected officials can be quite good at blaming others for their own failures (this tendency is by no means limited to British government). The EU is the convenient scapegoat in European politics, even if Britain has wide latitude to opt out of swathes of EU politics or even drive the EU itself as one of the largest members.
The other point you make is quite true: even Brexiteers didn't think they would win the referendum. And the problem is that, in unexpectedly winning, they wanted to lock their success in quickly, before anyone could discover the problems in their arguments. And when they themselves found the problems, well, see the point I made: it's not the Brexiteers' fault that they couldn't come up with a deal that the EU would like, it's the EU's fault that Britain is in this mess.
The other point you make is quite true: even Brexiteers didn't think they would win the referendum. And the problem is that, in unexpectedly winning, they wanted to lock their success in quickly, before anyone could discover the problems in their arguments. And when they themselves found the problems, well, see the point I made: it's not the Brexiteers' fault that they couldn't come up with a deal that the EU would like, it's the EU's fault that Britain is in this mess.