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Why we don't have 128-bit CPUs
(www.xda-developers.com)
This is a most excellent place for technology news and articles.
The ISA does include sse2 though which is 128 bit, already more than the pointer width. They also doubled the number of xmm registers compared to 32-bit sse2.
Back in the days using those instructions often gained you nothing as the CPUs didn't come with enough APUs to actually do operations on the whole vector in parallel.