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.
Public companies are also in competition with their peers to attract folks (read enormous investment forms) to buy their stock. So they want their “shareholder value” to be competitive. Shareholder value is at a high level the appreciation of the stock price plus dividends. So public company management is given the goal of increasing shareholder value. Which is the number that must go up. Otherwise those enormous investment firms will buy their competitor’s stock instead.