from talking to friends I think its more that there are really only 2-3 viable employers in the US, so there is little option to jump ship for higher salary. This keeps salaries artificially low. For example a friend of mine worked at raytheon, jumped to lockheed for a raise, back to raytheon for another raise, and then back to lockheed. You can only do that so many times before companies stop hiring you. Whereas in my field (IT consulting) there are an almost infinite amount of small companies to work for