Consoles are limited by power consumption.
PS3 at launch packed the fastest single PC GPU on the planet inside, the 7800GTX and the whole system pulled 200w of power while gaming, this was reduced over the years through die shrinks.
The 200w power consumption cap is still the killer and is what limited Sony's GPU choice for PS4 as it can use anything up to 180w+ while gaming.
7800GTX - 90w of power consumed
HD 7850 - 100w of power consumed
A top end R290x can use up to 300w+ of power by itself.
It will take YEARS for a ~100w card to offer the same performance that a 300w+ R290x offers today, for that high end performance to filter through to smaller, lower end devices.
So PS5 and Xbone.2 might only have GPU's no faster then the ultra high end GPU's we have today due to power demands and budgets.
The era of consoles launching with faster hardware then PC is truly OVER....