They could find an investor willing to buy them out. Doesn't have to be an IPO.

Still, if you are happy with the amount of money you have and you have a business you enjoy running, why sell? This is probably their view of it.

Yep, buyouts happen here more often than IPOs, just recently Socialbakers got acquired by Astute, and Integromat got acquired Celonis.

Also this is where Sun acquired NetBeans way back in 99.

Avast did their IPO in London. Most companies I have heard of were considering doing it in Singapore.

Why does it have to be in the USA? You can IPO at other stock exchanges. For example, CD Projekt Red, the game studio that developed the Witcher series and Cyberpunk 2077, is listed on the Warsaw Stock Exchange (Poland) and valued at ~$10bln (although the stock price has been really volatile recently).