Umm, because they won't work? i7 965 has no support for dual socket motherboards. It's the very reason there's a 55x0 Xeon line.
What you'd need for 3.2 GHz is a W5580 Xeon. It's about $1600 a pop. Newegg actually doesn't even sell the part. And its TDP is 37% higher than the stock MP CPUs.
FTR, Apple is charging you $2600 for the 2.93 GHz upgrade that cost them roughly 2x$1350 - 2x$375 (1k). IOW the markup is 33%.