Because the Lib/Con duality is a farce. Team A and Team B both work for team C. Team C is all about money and power. They are both Neocons. One plays the part of the carrot, the other the stick. They ramp up or down the fear of the stick or desire for a carrot to keep 3rd parties out.
Why do the cons complain? Because it's all disingenuous! Because it's all a piece of bad theatre. Everyone knows their roles and parts to play. The words are just lines. The play sucks. The play was meant to suck. It was written to suck and the casting requires good and bad actors alike to play their characters.
It is stupid, but so are we.