If the posterior pituitary only stores the adh (vasopressin) the hypothalamus makes, how come adh (vasopressin) doesn't go into the bloodstream when the hypothalamus is in tact?

Released by need: I'm not sure I understand what you mean. ADH (vasopressin) in fact, is secreted or released into the bloodstream from the post. pituitary. It is released according to demand and not before. So if you're asking why doesn't it "pour" out into the bloodstream then, that's the reason. There is a gating effect whereby NEED of the body (having to do mainly with sodium concentrations) signals the gate to open&close