Yeah but the question is "Why do they choose only Intel and nVidia?" and the reason is because nVidia pretty much owns the market and the mindshare. These aren't exactly cheap items no matter which way you go.
Not necessarily. If they had significantly better price

erformance, they'd probably entice a lot of people to switch. Granted, it wouldn't be everyone, but it might be enough to be worth it. It's not like they're going to lose money and the position that they're in is completely untenable as it is.
I keep hearing that nVidia has 88% of the market and that frightens me because at some point, AMD might just decide that Radeon is a failed product and stop producing them altogether. If AMD throws in the towel, what do you think that will mean for the market? At the moment, nVidia is willing to charge exorbitant prices even WITH Radeons competing with them. Can you imagine what the prices would be if there were no Radeons at all?
Jensen already tried to give the lame "Moore's Law is Dead" excuse which means that he intends to keep raising prices generation over generation. If there were no Radeons, how long do you think it would take before PC gaming became a boutique industry that only catered to the very rich? I guarantee you, if AMD throws in the towel, it's only a matter of time before an nVidia halo-level card costs $5,000USD because you'll have no other choice.
The existence of Radeon has made a lot of people spoiled because it makes their nVidia cards less expensive. However, if people refuse to support Radeon, it won't be there one day and when that happens, there's no going back and the people who have
only ever had nVidia cards will have nobody to blame but themselves when they can no longer afford to game on PC. Hell, I'd be willing to bet that a good number of people wouldn't be able to afford to game on consoles either because nVidia would own that market as well.
People have to look at the bigger picture because if they buy a Radeon today, they'll still be able to buy a GeForce tomorrow. The inverse may not be so true.