Part of it is the "second Einstein effect". Once you have one Einstein that works out general relativity, then there isn't a job left for a second Einstein. One other thing is that in physics, once you figure out some fundamental law, you are done. Once you've worked out string theory or quantum gravity, you let everyone know, and you are finished.
In economics, the rules change so often, that knowing how derivative markets worked in 1980 is only of marginal benefit to knowing the rules in 2012. So you have to rederive all your models every few months.
Has headed. The bus left decades ago.
The US became a post-industrial service economy decades ago. Citigroup has roughly the same head count as GM, and Morgan-Stanley has roughly the same head count as Chrysler. One thing that changes public perceptions of finance is the fact that finance is not unionized. When you talk about an auto company, the fact that you have a union makes people aware that not every in an auto company is a auto executive.
Because financial companies are tight lipped and non-unionized, the only people that people on the outside see are the top executives, which makes people assume that everyone that works in a bank is a managing director.