Because AMD was deathly afraid of AMD64 going the way of Itanium. So they went out of their way to make their architecture as similar to 32-bit x86 as possible,

Because AMD was deathly afraid of AMD64 going the way of Itanium. So they went out of their way to make their architecture as similar to 32-bit x86 as possible, right down to reusing the encoding. They also probably figured that they could reuse decoding logic.