When only 2 company make GPUs in any number it’s easy to control supply and keep stock tight or even create a perpetual shortage. aIts supply and demand, if demand out strips supply the price goes up, when supply outstrips demand the price goes down.
After the crypto mining crash we didn’t see a big fall in prices despite a huge oversupply of GPUs because they would rather store them in warehouses and drip feed them in than create and oversupply.