Soldato
Because it has to mess around with Windows, DirectX, abstraction layers, software drivers and christ knows what else getting in the way of its raw performance. Now take a locked spec system, code purely for that system and as close to bare metal as possible and you'd see significant performance uplift.
It’s gets you some. But it doesn’t get you much.
Look at the X1X for example. It practically uses an rx580 GPU. And compared to a pc running the same GPU, same settings, even with potato CPU like the X1X has they run almost identically.
4k 60hz is a reasonable expectation as the new Xbox will be twice the power. But it’s almost a given that there will still be graphical compromises.