Because, like charging into submitting A50 so handily taught us, "just getting on with it" really is the best route forward. Little chance that rushing such a huge decision could cause 3 years of gridlock and misery a second time round surely?
If there's anything this whole charade has taught us, from Cameron resigning immediately, to the rush of A50, to deals being bought to vote with no time to scrutinize, it's that rushing is a fucking stupid Idea. We're trying to extract ourselves from an incredibly complex arrangement, and dumbing it down might sound great for campaign slogans, but only serves to further frustrate an already frustrated electorate when they start rightly asking, "If it's so simple, why haven't you done it?".