Why is Charizard not a Dragon?

Why is Charizard not a Dragon? Dragon-type Pokémon are extremely powerful and have a much higher Defence than regular Starter Pokémon types. So, making Charizard a Dragon-type Pokémon would ultimately make him overpowered when compared to Venusaur and Blastoise, and that was not possible.

Dragon-type Pokémon are extremely powerful and have a much higher Defence than regular Starter Pokémon types. So, making Charizard a Dragon-type Pokémon would ultimately make him overpowered when compared to Venusaur and Blastoise, and that was not possible.
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