Because there is no better performance then Intel. AMD was a sort of let down.....
Aww come on give them a try! I own a Phenom II X4 955 that I love! I have the same motherboard as you and I can safely say I have never owned a better AMD motherboard. I was new to overclocking just like you when I got the sabertooth and 955 but with some help from friendly people on toms hardware I have my Phenom at 3.8ghz per core matched with a hyper212+. Not only now is my processor faster but it runs cooler than it did with all stock settings in the bios. With the newest bios update for the sabertooth 990fx it had my stock voltage at 1.39v at 3.2ghz now its at 1.34v 3.8ghz with lower temps now.
My advice would be sell the fx chip and get a phenom II X4 get it in the range of 3.6ghz - 3.9ghz per core and sell the 560's to get a gtx 680 (Wait a couple months till aftermarket cooling versions come out) and there will be no bottleneck