It is not about "legal obligation" towards share holder. It is about maintaining and boosting the "valuation" of the company. Why does it matter? Well, because

It is not about "legal obligation" towards share holder. It is about maintaining and boosting the "valuation" of the company. Why does it matter? Well, because the higher valued your company is, the more investment you can get, and with more investment, the better you can beat the competition. And if you don't do it (bad stuff), then your competition will do it and will ultimately will beat you. So it is basically the only way to survive. That is, keep growing. Stop growing, and you are on path to demise. No public company have the luxury of saying "Well, we have grown enough. Let us just focus on sustaining what we have got".