Why are Brexiteers so scared of a referendum on the EU exit deal

Last year the UK voted to leave the EU. No one knew what we were voting for, just what we were voting against. Many promises were made by the Brexit campaigners, the biggest one being the £350 Million a week for the NHS. Brexiteers campaigned for years for a referendum and in the light of the result, many have stated "the people have spoken" implying that referenda are the way to decide such things. They say that the "will of the people must be respected". But not enough to give them a say on the deal that is actually agreed. To me it is perfectly logical that Theresa May negotiates a new deal and then says to the British people "This is the best deal we could negotiate, do you want us to implement it?" Unlike the previous referendum, we'd know exactly what we were voting for. If the Brexiteers are right and the will of the people is that we go for it, then fine. But what if Theresa May negotiates a rotten deal that is not in the interests of the people. What if the deal will mean that the economy will struggle and rather than raising our living standards they drop, causing a drop in tax take and a funding crisis for the NHS. Since the vote, the pound has slumped and inflation is starting to rise. By the time May has finished her negotiations, the picture will be far clearer. Theresa May has put hard line Brexiteers in charge of the process. These guys have a blank sheet to get the nest deal they can. Surely every Brexiteer must believe that a second vote would be a formality with a great deal on the table? If they do they should have no issue and if they don't they are being cowardly and dishonest. It would give the Brexiteer ministers a huge incentive to do a good job. For me that sort of incentive is most sensible.