It’s simply a matter of economics and architecture. We may reach a point where GPU’s are able to pump out arbitrarily large resolutions such that adding more computational power does nothing to improve graphical fidelity. However, if that happens then GPU’s as a whole will begin to plateau as the market begins to slow its consumption of next gen cards as they become overpriced devices which offer only bigger numbers on paper rather than anything meaningful. Until then (and even then), PC’s will remain a free market of diverse and competing components which operate without fixed pricing.
When a new console is released, it is at best as good as current generation PC hardware. Consoles have typical life cycles of 5–10 years these days. That means that by the time a console is halfway through its life cycle, PC’s have had about 3 years of continuous annual improvements. Assuming Moore’s law, by the time consoles hit their stride, PC’s have quadrupled in performance since the onset of that console. It’d be safe to say that a gaming PC made 3 years into the console life cycle should be 4X the power of that console, but that ignores that consoles really target what would be mid-to-low-grade hardware performance out of a comparable PC at the time the console launches because consumers want a mid-to-low grade price tag.
So consoles will always be behind PC’s because PC gaming offers a market subset of whales where there exist some consumers who will happily spend 5X the cash for 2X the GPU power. This keeps hardware development on a continuous economic loop of research, development, and marketing of hardware. PC’s and consoles are very much opposite businesses because PC hardware makes profits, where consoles lose companies money. Xbox and Sony avoid continuous hardware improvement because a new generation is nothing more than a money pit which does nothing but keep them in the game. PC gaming culture pumps money into hardware and manufacturers actually make a buck, so they’re always aggressively developing and releasing new hardware.
Console hardware as a side effect will always be behind high end PC’s. The silver lining is that the largest market for gaming is consoles, so software will always be designed with heavy emphasis and prioritization upon that market segment. High end PC’s will always be better. However, consoles will always be good-enough for most games, and can sometimes be better value.