Microsoft just stocked store shelves with fresh boxes of Windows 11, and just like anything in the retail world, that means old stock, in this case boxes of Windows 10, are going to be pulled. Old stock, in this case potentially from Microsoft, its distributors, retail chains / outlets etc. are going to be sold / auctioned to 3rd parties who'll sell it on Amazon, eBay etc.
While you can still buy OEM Windows licenses for less than $20 -- licenses sold in bulk, e.g. to OEMs, are not anywhere near as expensive as retail -- this is not Microsoft approved and many people stay far away from such things. If you're one of them, this means you could get the official Microsoft endorsed product cheaper right now -- Amazon had a listing for $69 for Win10 Pro boxed, sold & shipped by a 3rd party seller. There is no real difference between Win10 & Win11 licenses BTW, and since the software in the boxes is already older than the latest releases, you'd likely want to download the latest Windows version of either 10 or 11.