@OP:
Several factors:
1) You cite PC/console multiplats, in this case PS4 and XBOne PC-like architecture makes a lot easier to choose any of them as native architecture and to port to the others with as little difficulties and compromises as possible.
2) The generous amount of RAM in PS4 and XBOne, and BD on both, besides helping point 1 too, allows more ambitious projects with less difficulties to squeeze them in the RAM available as it happened in 7th gen. It also makes possible to port large PC project that would be impossible to port to 7th gen without heavy downgrades and suboptimal cut into small pieces of the largest levels and it allows finer detail, both graphics and functional, allowing a larger number of interactive items at a time. It also allows to start PC/console multiplat projects that can use a larger part of PC's potential (possibly exceeding low end PCs' one, like unexpectedly showed Steam user clients statistics). 7th gen was powerful with little RAM, this meant long dev time and costs to try and fully use available computing power, while 8th gen makes a shorter performance generation leap, but RAM, despite growing by a relative factor similar to the past, 16x, grows by a huge absolute amount, and what's more, 32bit architecture OS architectures survival very long after HW switch to 64bit kept max theoretic usable RAM within 4GB for most games, except a few with 64bit enhanced versions and even fewer 64bit-only, so this time the RAM leap didn't leave consoles equipment behind PC average and it makes easier to fully use CPU and GPU power without the difficulties, longer time and costs of squeezing the necessary code to do it into little RAM and to have to smoothly load small chunks of large game levels at a time into it too, while on PC it can already be done more comfortably with shorter dev time and costs. This, BTW, this also allows to have larger viewing distance even with very detailed worlds, so fogging doesn't partially ruin all the efforts to make graphics as good as allowed by the GPU anymore.
3) Points 1 and 2 can persuade easier devs to port large projects previously meant as PC-only.
4) 7th gen lasted more than expected, so probably 3rd party mid and long term plans were readier than usual to move to next gen.
5) 7th gen home and portable console markets leader killed its last gen platforms very early, this probably gave 3rd parties largest projects the green light to start moving without expecting PS3 and XB360 would have survived so longer past their respective peaks.
And there can be many more reasons that didn't spring to my mind.
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