I have exclusive information about Pokémon Sword and Pokémon Shield.
Unlike the Charmander line, the Bulbasaur line and Squirtle line are not found in the Galar region. This is explained through the in-game lore and the concept of 'Starter Pokémon'.
The Charmander line has been found in the Galar region since a very long time, but the Bulbasaur line and Squirtle line have never been seen by the natives of the Galar region. Hence, the Charmander line is considered as a regular Pokémon, and not as a Kanto Starter, similar to how Spheal, Sewaddle, Seedot, Poliwag, etc., are considered as normal Pokémon.
Regional Professors choose what evolutionary lines to distribute as 'Starter Pokémon' based upon their rarity and obviously, their type.
Hence certain species are considered as 'Starter Pokémon' in certain regions, and cannot be obtained in the wild.
To go along with this theory, it is shown that wild Charmander are very common in the Galar region.
