I've come to think that it's rather the expectation that they are expensive. It is expected by employers as well as the old employees themselves, so if they would demand a lower than expected wage it would come across as fishy (did he serve time? an underachiever?).

Another thing might be that with experience comes less naiveness. As you become more resilient against corporate BS you become equally unformable (as in "Thanks, but no 'chakka' for me please").