4090s aren't really normal either. How many people have dual GPUs? I don't think it helps much with games last I checked so you'd only buy 2 for AI.

It's more about what's possible to build. Dual 4090 or 3090 is possible to setup without hassle. Beyond that not really because it'd be above home power socket rating, not possible to fit on the board and case etc.

It's true you can also build dual A6000 with 48+48 = 96GB VRAM also, but that's $10k+ setup just for GPUs on legacy generation.

There’s the physical hassle. It was very difficult for me to fit 1 3090 in my case.

Yes, watercooled variants are better for dual setup (at least one, better two).