It in no way justifies it but this is usually because left hand doesn't know about the right something something good engineering. You have the site that's independently run and exists as nothing but a client to the other system that handles inventory and fulfillment blah blah business! And that system isn't made in-house so you're at the whim of whatever order submission "thing" (because it's not always an API, the horror) it exposes to you and sadly a lot of them the only way to see if you can actually complete a purchase is to try and see if it errs.

Suplemental: Stop listing things in product pages and search results, just to click through and find it's out of stock.

Been searching for a particular pair of skis recently, and so many places listed them, only to click through and find they have no stock in any size. Complete waste of time.