Speaking from a strictly game cannon perspective, the fact that you do not meet a Pokemon in the wild does not mean that it doesn't exist in the wild. While playing Pokemon Red, you don't meet wild Persian and Meowth, but, that's not to say that they don't live in those areas [Pokemon Blue proves that they do,] all that this shows is that, in your journeys as a trainer, due to fate or destiny or luck, you have never encountered one of these Pokemon.
The same can be said of the starter Pokemon: [or indeed, any real life animal which you have not encountered] The fact that you passed through their habitat and did not meet any of said species does not prove that said species does not live in the aforementioned environment.
As it stands, the only starter Pokemon you meet in the game happen to belong to other trainers; that's just how your adventure has worked out.
Looking at the situation rationally, it's fairly safe to assume that Bulbasaur, Charmander and Squirtle are all fairly uncommon in Kanto, to extent that numerous trainers may never see one in the wild and that those given out by Professor Oak are obtained through breeding. [hence the portrayal of all starter Pokemon in the anime to be babies.] I can't see a reason as to why this shouldn't apply to every region. [Except for the fact that, speaking once more from a strictly game cannon perspective, we don't know that either Professor Elm or Professor Birch hand out starter Pokemon to newbie trainers; Professor Elm was studying Chikorita, Cyndaquill and Totodile, and gave the Pokemon to your trainer so that he could safely run an errand for him, and it's never implied that Treeko, Torchic and Mudkip aren't just Pokemon that belong to Professor Birch.]
... There, I think that clears things up. ^^
As an aside, where does it say that starter Pokemon come from exotic regions? None of the Professors that I've talked to say that. >.> <.<
Also, and this is specifically aimed at the two posters above, don't just make stuff up, don't try and answer these questions if you don't know what you're talking about.