Because Nvidia's current prices are massively inflated, that's why. Nvidia's current performance level should not cost what it does right now, or to put it another way, do you really think that Vega 64 +15% performance should cost £450 near the end of 2019?
I agree the prices are ridiculous, but why some people think AMD should be half the price ill never understand. This tends to be the mindset of people who will never buy AMD cards and wont pay the inflated prices of Nvidia. So rely on the cheaper AMD cards to bring down Nvidia prices.