Gen 1's goal was to catch them all, and the games are designed around that goal.
Think of every single-stage mon in gen 1.
Farfetch'd, Lickitung, Mr. Mime, and Jynx can only be gotten through in-game trades.
Chansey, Kangaskhan, Tauros, Scyther, and Pinsir are found in the Safari Zone, requiring you to play a minigame to get them.
Porygon is found in the Game Corner, requiring a different minigame. Scyther and Pinsir can also be found here.
Aerodactyl is gotten by exploring two optional areas.
Snorlax only gives you 2 chances to catch it. Period.
Tangela and Electabuzz are both in easy to skip areas, and while Magmar is in a required area, it's a rare encounter and you likely have repels at this point.
Articuno, Zapdos, Moltres, and Mewtwo are legendaries that require you to go out of your way to reach.
Single-stage mons are just meant to give you more mechanics to engage with with the intent of catching them all. Same as the different evolution methods of trading and stones.
Only exceptions are Onix, designed solely to be a first boss, and Ditto, which is a gimmick/meme.
TL;DR: It's not designed to be good. It's designed to be another box to tick off on your Pokedex. At best, it's a fire type in case you didn't already have one.
