I've said it elsewhere, but I STRONGLY suspect the problem isn't so much AMD and Nvidia, as it is the third party AIBs making bulk deals with miners. (Actually, AMD and Nvidia almost certainly allocate some percentage of cards direct to miners as well these days, for the same reasons as I'll outline here.)
You can't tell me that if someone approaches Gigabyte, Asus, MSI, Sapphire, or whatever other graphics card company you care to name and says, "We'll pay you 50% more than MSRP for 5000 graphics cards -- and we'll pay you in cash, right now," that there won't be some sort of back room deal struck. If you have enough money, you can make the contacts and deals to get what you want. Miners are willing to pay scalper prices direct to the graphics card manufacturers. And the AIBs normally sell the cards to a distributor who sells to a retail outlet who sells to the customer -- so basically subtract 10-15% profits for each level of sales.
Example:
A) AIB sells 5000 RTX 3080 cards to a miner at $1000 each = $5 million to AIB.
Versus:
B) AIB sells 5000 RTX 3080 cards to a distrubutor at $500 each = $2.5 million to AIB. (Then the distributor sells to retail chain at $625 each, and the retailer sells to customer at $750 each -- or sells via eBay at $1200 each.)
Considering the cost of the cards to the AIB is probably around $400-$425 (GPU, RAM, PCB, cooler, packaging, etc.), the AIB makes perhaps $500,000 in profit on option B (and still needs to pay salaries and such), compared to making $3,000,000 in profit on option A. There's no way you have a company choose option B if it's given option A -- it just needs to maintain the facade of doing option B, for at least some percentage of cards, while selling as many cards as possible via option A.