The caricature of investors as "the wealthy" is not connected to reality. If you are interested in having a retirement, you have a stake in the game - even if your entire plan is to rely on social security, someone has to make the money that will be taken and given to you. If you have an interest in a high standard of living or low cost of living, efficient investment decisions are vital to you.

When you hire someone, whether to mow your lawn or run your fortune 500 company, you want their work effort to be aligned with you objectives. Ideally, every employee you hire is aligned with your interest, but that is impossible. This is a hard problem with a huge body of research behind it. If you would like to learn more, look up the "agency problem".

With regard to the ability to easily change investments - that is simply not true for the massive companies you are talking about. As an individual investor, you can shift capital around easily because your decisions have a negligible impact on the market as a whole. However, over 70% of the stock market is held by institutional investors, whose decisions significantly impact market prices.

Stock prices reflect the long term prospects of a company. Shareholders are merciless in punishing companies that optimize for the short term over the long term.