I don't think they are remotely the same games each time. It has the same formula sure but all video games keep the same formula. The plot, the characters, the setting, the pokemon, the catching style, the accesories and UI all change. The only thing that stays the same is that it's a turn based RPG where you catch pokemon. They do just about as much leg work as most Turn based RPG franchises do now. Same as a Mario. It's the same old formula we know and love but not by any means are the games the exact same.
Same applies for Call of Duty. I disagree very much with the notion that it's a rehash. I've heard that statement since 2007 regarding Call of Duty. The game is over a decade old and still sells more than almost any other game. And it's because it changes enough to warrant another buy. Call of Duty changes immensely every year which is why people still come back. Outside of becoming a tennis game, they do all they can to make it feel fresh. The setting, theme, guns, modes, era, war, costumes, story, plot, side modes, UI. All change. The only thing that is the same as the old call of duties is that it's a first person shooter.
Theirs not much else they didn't change over the years. It changes about as much as Mario Kart does. But Mario Kart doesn't get the same hate because it's Mario Kart. Picking up Mario Kart 8, outside of minor nuances, it's Mario Kart Wii. But it has new tracks, new power ups, new kart customzing. It's very much different.
Last year COD tried the royale thing. This year they are doing hoarde modes and going full ground war with huge lobbies like battlefield. They heard for years that every call of duty was the same. So then they changed it to sci fi jet pack crazy stuff with wars in outer space and people got mad. Saying they wanted the old COD back. So they try to stay more grounded lately with WW2 games and now people are saying they are being boring and predictable again. I don't envy their dev team. Balancing on that tight rope must be draining.
As a whole, I think it's still popular because it's still a great game. Same reason you can repackage Tetris a bunch of different ways and still sell copies. It's a timeless classic that all ages can pick up and enjoy
. And so long as Pikachu exists, little kids will always want a Nintendo in their hand lol.