Brexit is just too difficult and complicated. Our politicians are not used to dealing with this kind of thing. It needs a fantastic team in the background getting to grips with the issues, and great leadership to come to decisions about what to do.
I imagine there is a decent backing team, but the leadership is entirely incompetent.
Most of the government negotiations I have worked on have lasted for ages, with the leadership team always revisiting decided items once they actually grasped the ramifications of the decision they had made. And to say that Brexit is more complicated is an understatement!