It's a combination of things you guessed (pricing tricks/marketing) at plus, real-time anything is very hard especially when you're making an offer to consumers. Goods can move quickly and purchases occur at the same time (many in fact) and it depends on the location an store house of goods. If you don't have tight controls around location, order times, and a host of other things, it's difficult to reliably give that information to an end user. I can see it being a bit easier for a warehouse only or B&M online item where the location and the product are very specific (we have 5 TVs at the DFW location) but get hairy when factoring in entire regions or product that is on various statuses but you can still deliver by a certain date. You don't want to over promise and constantly disappoint/refund customers or under estimate and not sell enough. Logistics, inventory tracking, and real-time data are hard especially all together.
Keep in mind I really have no idea what I'm talking about...