Because that would be another useless warning. I write

    for ($x .. $y) { .. }
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quite often, knowing that it will not do anything if $y < $x.

    @a = 1 .. 3   # 3-element list
    @a = 1 .. 2   # 2-element list
    @a = 1 .. 1   # 1-element list
    @a = 1 .. 0   # 0-element list
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The latter to be a 2 element list would be very surprising (and would require a lot of extra code in my programs).