Power, Temperature and Noise
Performance may well be similar to the GeForces, but power consumption is not. Nvidia's Pascal design is fundamentally more efficient, and is able to push out the same kinds of framerates whilst using 25 per cent less system-wide juice.
AMD, therefore, is a distant second in any performance-per-watt metric.
A by-product of having to tame more power is temperature that isn't as good as the competition, either.
And finally, that reference design does nothing to endear itself to gamers who want quiet PCs.
The comparison EVGA GTX 1080 card performs at about the same level as the reference RX Vega 64 but is much, much quieter, cooler, and far more power efficient. Full-on performance is only one facet of a card's overall attractiveness.
The likes of Sapphire, MSI, PowerColor and Asus will likely use extant RX 580-class cooling on their RX Vega designs, meaning a better all-round experience than the reference models.