Because it appears that some Brexiters think that Brexit should be easy. The EU created a common market based on a set of rules. We want access to that market but not to have to abide by their rules and not pay them any money and if May can't achieve that she is incompetent.
Honestly, you just couldn't make some of these people up. Brexit was always going to be enormously difficult and painful and we were always certain to be in a worse position in the short term; and if you didn't realize that you probably shouldn't have been allowed to vote.