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.
One aspect I haven’t read about: competitive pressure and economics of scale.
So, imagine two carpenters: they both produce one chair a day. They sell it and can sustain their families with that. Now the one carpenter works a little overtime and uses sharper tools: he’s able to produce two chairs a day. He still needs only to sustain his family, so he could sell the chairs at 50% discount. But he goes for 75% of its original price. Still cheaper, he has more.
Everybody wants to buy those chairs now: they’re the same, but one is way cheaper. The other carpenter loses business, he can’t sustain his family anymore, because he needs to sell one chair a day at least. To keep up, his business needs to grow now.