> I don't understand why they don't just hold another referendum and undo everything.

I see you follow the EU model of democracy; keep putting matters up for a vote until you get the result you want.

Despite what you may have been led to believe, there's still a substantial number of British people that reject EU membership, and this goes beyond the "racist" and "economically illiterate" arguments that remainers typically fall back on. I would suggest you do what your predecessors did not and consider the consequences of losing.