the reason is just the same as gyarados
You can find Charizard in Fireflow Volcano. Once you reach Research Level 3, you can find it flying around in the final chamber on the regular route. It is also fighting with a Tyrantrum.
Source: https://gamerant.com/new-pokemon-snap-every-pokemon-fireflow-volcano/#:~:text=Charizard - Once players have reached,for some pretty cool photographs.
