Their name rthymes with "Wait Models" for a reason...
I believe they often do not actually order the item until someone orders it from them. Then they place an order with their distributor, who in turn ships the kit to GM who then ships it to you once they have all the pieces of your order in hand. Some things they do have on hand and will ship an order fairly quickly if everything is in stock and on hand. But if not, you play the wait game until all the components of your order are on hand and ready to be shipped to you.