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OK lemme explain to you zoomers what Pokemon was about in its inception.
I don't know if nowadays with the internet being so accessible kids do the same, but back then collecting shit was the cool stuff you did. Sticker Albums with pictures of your favorite football players or cartoon moments, trading cards, toy lines, etc. Collecting stuff was a kid's pass time, and in Japan collecting bugs (namely beetles) is a somewhat common things for kids to do. Enter Pokemon, a game based on this concept, naturally it swept the world like a wildfire. The goal of Pokemon was for it to be a social game that required you to work together with your friends in order to complete the Pokedex. It wasn't a game you were supposed to replay often, it wasn't a game you were supposed to take seriously when fighting your friends with your mons, it became those things way after the appeal of collecting them all faded away around gen3 when it became clear that catching them all was going to be impossible for the average person since Gamefreak would pull the rug and add newer mons. The game was basically a virtual sticker album and the Pokemon were your stickers.
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The thing with collecting stuff is that naturally there have to be different degrees of rarity, Gen1 had trades and the Safari Zone as a way to gate keep you from certain mons. You wouldn't know what a Dratini was unless you decided to randomly use the Super Rod in the Safari Zone, you wouldn't know what a Mr.Mime/Jynx/Farfetch'd/Lickitung was unless you spoke with random NPCs and traded with them, and you'd never get a Golem without interacting with your friends, etc.
Word of mouth was also fucking HUGE when it came to collecting. How to get X, how to get Y, kids would help each other all the time and that's how weird rumors like the Mew under the truck among others popped up.