Loads of reasons. The shareholder theory of corporate governance is actually not very old.

And what other theory is there? The only two I know of are the shareholder theory and the vague "Capitalism bad. Shareholder bad." theory, which isn't actually a theory, but a complaint.

Your question is a reflection of just how engrained the Friedman doctrine has become in business. Milton Friedman introduced his theory in 1970, but it really got a boost in the 80s. First in 1981 when President Reagan named him to his Economic Policy Advisory Board and again in 1988, when Reagan gave him the Presidential Medal of Freedom and the National Medal of Science.

There are still many competing theories of business ethics, but the Friedman doctrine is what drives corporations today.

Was anything form Friedman actually proven correct or worked on practice?

I don't understand how he became such a big name.

EDIT: Hum... Do the people downvoting have some answer?