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1) they don't, Blue and some of Sinnoh used multi-type teams
2)it's a thematic choice, and they aren't intending to be unstoppable; the gym leaders have multiple Pokémon teams to test their challengers' knowledge of types and strategy, while also using a type that traditionally is represented by the town, but in some more modern locations is represented by the individual, or in fringe cases is simply due to personal preference.
