there is no too many jobs need highly trained CS graduates

With open source code around, most of CS graduates can do the same work. Unless elite college graduates have some special skills, they do not have special value for highing pay. Wall street needs their graduating colleges as brands to attact customers, so they likely pay higher salaries for graduating college brands. Basically, people trust their money is managed by successful people, brands of graduating colleges are one of them to associate with the trust easily.