For somebody who go's on so much about evidence and facts, you do spout a lot of tosh in here, posting points of view and passing it off as facts.
The simple truth is the politicians were arrogant enough to believe they were good enough to see this through without expert advice/support. Had the experts been approached I'm pretty sure that, irrespective of whether they supported leave or remain, the majority would have jumped at the opportunity to be part of the negotiating team. Not only would it be great on their CV, it would also give them a chance to influence proceedings from the inside.