It's the 'movie' version of negotiating that plays to a populist crowd.

It intersects with other 'tough guy' identity tropes that are being (successfully) used in domestic political battles to gain power, and then are repositioned to be used in foreign relations with predictably terrible outcomes.

The problem is the audience of the negotiating stance isn't the counterparty, it's the domestic base who elected you because they were fed up with nuance and are suspicious of the counterparty.

This isn't just a Brexit problem, but is much more widespread.

I don't know how you'd be able to align incentives any worse.