Because it's designed not to have too much idle states to begin with? That's what the shared front end is all about,maximizing the utilization of all hw execution units(2 int and/or fp).They will actually use a form of SMT in the FP/simd unit anyway.And if they would try and opt for a sort of SMT in int execution units they would have to deal with a whole another level of possible complications and design complexities. It's not impossible but unlikely to be seen even in Bulldozer successor on 22nm.
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