NVIDIA makes probably about a 40% margin on chipset sales (They sell the RAM with the GPU as a set)
The card vendor may make another 10->25% or so depending on scarcity and model rank (higher end models make more margin) That's 10->25% nvidia isn't making.
There's zero reason why they couldn't pick one supplier and say "We'll give you 100% of all mfg, if you give us a steep discount to make OEM cards for us with our branding." To the big mfg (like Sapphire, or EVGA, Foxconn, etc...) that's a workable deal because while they make less per unit, they capture a majority of the market, and NVIDIA bears the risk of selling them, thus netting a higher net profit, while also handicapping their competition. At one time NVIDIA owned all their own assembly plants. But competition was tight and they were forced to divest themselves of that. There is no competition any more really. (Except AMD)
This puts more margin in NVIDIA's pockets while putting smaller card vendors like Zotac, ASRock, etc out of the GPU business.
Yeah, ATi used to make their own cards as well. IIRC, they stopped doing that when they struck a deal with Diamond Multimedia to buy their pro-level FirePro line. Part of that deal involved Diamond selling ATi Radeon GPUs under their own name. I think that made them the first video card AIB partner because not long before that, they were producing Diamond FirePro, Diamond Stealth and Diamond SpeedStar video cards of their own design.
You're right that nVidia COULD give all their business to one AIB partner but it would be detrimental to them. Suddenly this partner will have no competition and that is detrimental to innovation. Let's go with the obvious choice, EVGA. Suddenly, you'll have names like Palit/Gainward/GALAX/KFA2, Sparkle, ECS/Elitegroup and PNY joining names like XFX, Sapphire, Club3D, HIS, BioStar, Diamond and VisionTek on the Radeon side. They'll compete with each other like mad which will drive prices down and board innovation up. Zotac will cease to exist because they're just Sapphire under a different name. Add in the former dual-wielders like ASUS, Gigabyte, MSi and ASRock. Suddenly you've got A LOT of competition for the smaller Radeon market. This will cause the Radeon market to swell because prices will be far lower than they already are because of the number of companies trying to compete in the Radeon space.
If nVidia decides at some point that they've become tired of EVGA, none of the other board partners will want to deal with nVidia again after seeing how well they get treated by ATi in comparison. This is why ATi never had a major AIB partner jump to nVidia even if they don't sell as well. It's also why nVidia lost a MAJOR AIB partner to ATi in the form of XFX. I remember having an XFX GeForce card back in the day and then an XFX Radeon HD 4870 1GB (which became TWO 4870s). I didn't know why XFX would do this because they were either tied with or just slightly behind EVGA in the nVidia board hierarchy, making GeForce cards and nForce motherboards. They became disillusioned with nVidia and started making ATi cards as well, starting with the HD 4xxx series. Then nVidia threw a hissy fit and refused to sent them Fermi cards. XFX shrugged, went all-in with ATi and has been quite happily making only Radeon cards ever since. Here I still remember the XFX GTX 280 Black Edition (those were a big deal back in the day).
No matter who nVidia deals with, they end up pissing them off (they even pissed off EVGA with the Founder's Edition cards). No matter who they choose, sooner or later, they'll be at odds with them just from nVidia being nVidia. LOL