Because manufacturing isn't the only cost. If AMD took the chips it couldn't sell, it has to pay for logistics and storage and oversight and accounting and, eventually, disposal. It costs more to actually do it than to just pay GF in cash. It also #$%$ GF management off less and allows them to allocate resources to something else, allowing them to make more money.
So it's extra cost for AMD, and lost opportunity cost for GF.
But of course you clowns think like ghetto dwellers, with your sell-and-lease-back parties and your cash-secured lines of credit and your consoles are better than computers mindset.
I will never have to cover. And I may never see any of you idiots admit that.