I think that Niantic switches on/off the wild alolan every so often. Usually during limited events. I may be wrong, but I don't think they are always available (I didn't do any research to confirm this, just observations). But even if they are, their rarity is higher than larvitar or dratini. That makes them pretty much unavailable in the wild.
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Reaction to the article "Revisiting team with ultra bonus event": Alolan Exeggutor *IS* out there in the wild
"Alolan Exeggutor has only been available through events and raids."
That is not true. I saw an Alolan Exeggutor in the wild weeks after the event and others have also mentioned this. It's just rare and doesn't nest, like many evolved pokémon.
Granted it probably won't spawn during this ultra bonus Kanto event though :)
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This is absolutely possible. But this still makes the assertion "Alolan Exeggutor has only been available through events and raids" wrong. "pretty much unavailable in the wild" doesn't mean much: it's like saying "pretty much impossible", it still means it's possible. I've met more wild Alolan Exeggutors outside events (1) than Unowns (0).