It's just that you'd rarely encounter an isolated e pawn. Isolated d pawns are common because the c and e pawns are pushed -- isolated e pawns would involve pushing the f pawn. In a bare bones structure you're right it's the same -- it's the kings on the board that make the e,f,g,h files a bit different from the a,b,c,d files. So that even if you were to encounter an isolated e pawn, the advantages and disadvantages would likely be a bit more dynamic due to how the position was reached and where the kings castled.
IQP vs IKP
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I've done a lot of reading about the IQP and the advantages and disadvantages of having one, and it seems that if you have to have an isolated pawn, the d-file is usually the best spot. My question is why there isn't an equal amount of talk about an isolated pawn on the e-file. It seems to me that they'd both share the same advantages and disadvantages since they are both central.