Probably because most 1.e4,e5 pawn structures tend to develop into other common pawn structure types...open center, open e-file, and King's Indian/Spanish Game pawn structures are some usual ones. The Russian Game, for example, is an opening based on an open e-file pawn structure.
Also, I'd argue that the division of pawn structures into themes or classes is somewhat arbitrary (I suppose some better players might disagree with me on this), which could also possibly explain the discrepancy.
Critical Pawn Formations
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I've been reading about openings in wikipedia, particularly pawn structure. Andrew Soltis presents 16 major themes in pawn structure. How come he does not mention Kings pawn openings where black plays e5?