Why do new military recruits take the Oath of Enlistment to get into the DEP if they can quit at any time?

From what I understand, recruits in the DEP (Delayed Enlistment/Entry Program) can quit at any time and get out of the DEP before they are shipped off to their IADT (Initial Active Duty Training) whether it is basic training or boot camp or whatever. So why do they have new recruits take the Oath of Enlistment and then the recruiters will tell them they are bound by contract since they signed and took an oath, but they can still back out of it. Why don't they make them sign or take the oath the day before they ship off to training or something like that? Seems weird. Does anyone know why it's set up like this or why?