Both of them are free, though. The former one only got this push due to streamers suddenly playing it together. These are kind of "meme of the moment" games more than anything, in the same way VR Chat was a bit ago, or Five Nights at Freddy's.
Fall Guys was "free" on PS4 at launch in the sense that it was included in PS+.
Among Us is just really cheap (free to play on its original mobile platforms, I believe, but that's not really the source of its current moment of popularity).
Plus is a strange case because you need it for online play and some other PS features like cloud saves, so many people have it regardless of what games are on offer. Let's just say that a large number of PS4 players got it at no marginal cost.
