I think the point is that the third party library between the Xbox, PlayStation, and PC is very similar, almost to the point that you COULD only own one of them and have access to most multiplat games. No need for PC. Whereas owning a Nintendo console, you only have access to a handful of third party ports and it's exclusives. I'm not sure why you think people need a PC no matter what for multiplat games, because it's really not true.
Generally because PC is a better platform in almost every respect to play your standard multi platform games and shoot bangs, with the additional benefit of a generally better experience, an open platform, cost savings and having even more exclusives than a Nintendo platform (which tend to have the most exclusives on consoles) by orders of magnitude.