This is a good callout, unlike manufacturing the supply chain is almost universally vertically integrated for large software projects. While it's possible to make a kit car that at least some people would buy, most of the big tech companies have reached the point of requiring hundreds of engineers for years to compete.

Caveat that time has shown that the monopolies tend to decay over time for various reasons, the tech world is littered with companies that grew too confident in their monopoly.

- Cisco - Microsoft Windows - IBM

etc.