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.
Under capitalism, companies do what their owners want them to do. The owners can choose to try to grow, to shrink, to sell, or to close.
Publicly owned companies have shareholders, and the shareholders usually want the company to grow so their investment grows. Shareholders can have other values, but anyone can become a shareholder.
Under non-capitalist systems, the government might own some or all companies. Then the companies do whatever the party in power wants. The party in power probably doesn’t have time to run all the companies, so they give some level of independence. They can reign that back whenever they like.
The most common motivation in non-capitalist systems is probably greed and growing personal wealth of party leaders via corruption under that system. Luckily, the people can vote in a different party and/or protest against party corruption except in all real-world cases, where that is banned or suppressed.