Edit: This question attracted way more interest than I hoped for! I will need some time to go through the comments in the next days, thanks for your efforts everyone. One thing I could grasp from the answers already - it seems to be complicated. There is no one fits all answer.
Under capitalism, it seems companies always need to grow bigger. Why can't they just say, okay, we have 100 employees and produce a nice product for a specific market and that's fine?
Or is this only a US megacorp thing where they need to grow to satisfy their shareholders?
Let's ignore that most of the times the small companies get bought by the large ones.
That is a common misconception, very often spread all over the place on Reddit. There is no such requirement.
And corporate case law describes directors as fiduciaries who owe duties not only to shareholders but also to the corporate entity itself, and instructs directors to use their powers in “the best interests of the company.”
Serving shareholders’ “best interests” is not the same thing as either maximizing profits, or maximizing shareholder value. "Shareholder value," for one thing, is a vague objective: No single “shareholder value” can exist, because different shareholders have different values. Some are long-term investors planning to hold stock for years or decades; others are short-term speculators.