A fair point is that yes, starters delivered as Starters
are bred and trained for that purpose. For that matter, they wouldn't necessarily be the 15 Fire/Water/Grass families only, but also theoretically apply to starters and mons passed down by family heritage, as at least two anime episodes (one related to Brock) suggest.
The anime and manga have wild starter lines, though they are more or less implied to be scarce, or hard to find (early Johto at least; dunno about early Unova). Or particular note, starters in the games are not delivered only as Starters but also as gifts or plot prizes.