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If you really think about it, Missingno was lightning in a bottle. It only existed because the game interpreted its sprite, stats, moves, and even the seen/caught flag using random locations straight from memory (e.g. Biker parties or your bag) all while using a placeholder name. On top of that you could also find it in RB without cheating which meant rumors would spread about how dangerous/rare it was. Future gens also had more sophisticated ways of handling glitchmons so while there are some that slip through the cracks (Question marks, Gen 4 hybrids, etc.) they're not as batshit insane nor accessible as a Gen 1 glitch.