...and why did EVGA withdraw from the GPU market altogether rather than pivoting to making AMD/Intel cards, if Nvidia was truly the problem?

I think EVGA’s withdrawal had to do with how impossible it was to compete with Nvidia’s first-party cards with the terms Nvidia was setting for AIBs. Other card makers like Asus have several other flagship product lines to be able to sustain the hit while EVGA’s other products consisted of lower-profit accessories.

They may have seen AMD selling their own first-party cards and anticipated AMD eventually following Nvidia’s footsteps. As for Intel, at that point they were probably seen as too much of a gamble to invest in (and probably still are, to a lesser extent).

Because GPUs tend to be low margin to begin with. Nvidia putting all these restrictions on them meant running at a loss.

This compares with PSUs which apparently have a massive margin.

EVGA might come back to the GPU manufacturing space with AMD eventually but Sapphire and Powercolor already fill the niche that EVGA filled for Nvidia cards (high build quality, enthusiast focused, top of the line customer support, warranty, and repairs). So it probably was just not worth picking that fight when the margins aren't really there and AMD is already often seen as "the budget option".

If AMD manages to pull a zen style recovery in the GPU segment, I would expect a decent chance of EVGA joining them as a board partner.