A lot of games implement physx even if you think it doesn't.
Nvidia already released the API and physx source code in 2015
Since physx can run in software mode on AMD, it also allowed developers to use physx and learn how to adapt it to run on AMD cards, and they did so in many games.
A lot of current gen physics in games is in this way or another, based on physx. So even if you "never saw the difference", in many cases, is because you watched physx without knowing it.
While it was not originally from nvidia, the fact that they bought it and pushed it, made a lot of difference in game development over the years and how people "never saw the difference", because they have no idea how technology evolve.