This is a simple matter of people doing what they are incentivized to do. The vast majority of directors have incentives tied to short term gains (short term in the sense of a company - quarterly or yearly), and so their actions are to seek short term gains. They might say they are required to act in the best interest of the stock holder... but it just so happens that they have compensation which is tied to short term gains in the stock price.
I don't believe the conventional wisdom that the stockholder only cares about short term gains. I see, over and over, share prices punished because the stockholders do not see the company doing well long term, and vice versa.
