My guess: Because it's non-obvious that, unlike Seq.zip, a Seq.unzip would have to cache the sequence.
However, if one has acknowledged this fact, a Seq.unzip could be useful for preserving laziness, e.g. when working with infinite sequences.

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module Seq =
    let unzip seq =
        let seq = Seq.cache seq
        Seq.map fst seq, Seq.map snd seq