Graphics card manufacturers believe that selling high-memory consumer graphics cards will affect the market for commercial computing cards, so they will not do so, that's all.
Just look at how much trouble Intel has had breaking into the discrete GPU market or even just how hard it’s been for AMD to compete with Nvidia even with decades of experience in the market.
And if some newcomer could make a competitive GPU with large memory capacity they’d be crazy not to sell it at datacenter prices, maybe just undercutting the others but a few grand but still way more expensive than any consumer GPU you can buy today, even a 4090.