It’s not that you play one game exactly. The Pokemon games are about collecting and raising monsters. Each of those monsters have some unique stats and metadata.

Each time a sequel comes out, those games usually give you a way to transfer creatures from the old game to the new game (it used to be by linking games directly, now I think they have a side app).

Even though each generation brings kind of a totally new game, in same ways they have aspects of expansion packs. You can keep using the monsters you had before and building a bigger collection.