Because
Amazon is full of terrible sellers? I mean they are a lot of them. If you get a bad seller they don't care about you in particular. I agree it would be easier to just send a message but you can't even get major retail companies, including Disney to do that, in a timely manner or at all to begin with.
As far as costs to send back it all depends on what the item is with its exact costs. I've had things that Amazon deemed cost more for me to ship an item back than to just let me keep it (that one was a cheap shirt from Chinese sellers). I've had Amazon deem it cost more to have me send something back to them than it was to just send me a replacement one (which meant I had two of them). I've had Amazon want the item back before sending me a replacement, etc.
It also depends on where the item was shipped from. If it says "Fulfilled by Amazon" it means the 3rd party sent the items to a warehouse for storage and Amazon shipped it out which could be either a seller issue (wherein the inventory was incorrect in the system) or an Amazon issue by the person who fulfilled the item. If it was shipped directly through the seller then it's on their end
I'm just spitballing a potential reason. That's why I asked about the delivery time with the correct item you wanted to see if it was something that used to show X delivery and now all of a sudden is much longer (which would be a potential sold out situation). And there's still the possibility that it's a display glitch, which is why you're waiting to see what you get in hopes that it was.