Git didn't exist, SVN didn't exist before 2000. Common languages in use today didn't exist or were in their first versions (go, python, java, etc...). Most of the unit testing frameworks and IDE that are taken from granted weren't invented yet.
There were different expectations back then. No software company should be running in 2020 with no version control, no unit test and no CI.
Java was very big in the Enterprise by 2005 and yes there were plenty of IDEs back in 1995. Amazon I believe was originally built on a C code base.
There are plenty of companies that have no automated unit tests and do quite well with manual testing.
But we aren’t just talking about source control and unit testing we are talking about code quality.
Code quality is limited to the tools available at the time. It was an uphill battle to preach for (automated) scripted builds or any form of (automated) testing without having the later tools/frameworks/infrastructure at disposal.
Funfact: The java compiler was still fixing "bugs" in 2016 to be able to produce an identical JAR between builds.
Nowadays if an intern opens PyCharm. He can get right away a lint report on the current file and entire codebase, finding potential bugs and questioning why is this thing not running automatically on commit. This pushes quality up organically quite a bit. By comparison doing embedded C++ development in the 2000's, I can only recall of one company having a static code analyzer, that costed no less than $50k for a handful of small projects (they charged by line).
C is not a new language. People have been writing large well structured C code for decades. The first C linter was written in 1978. Even PC-Lint was written for C in 1985.
Counterpoint #2: Amazon's code base was a monolith and highly dependent on Oracle. The rest is history.
The order of priority is to have a product that customers want, convince investors to give you more money to continue your growth and improve your infrastructure and hopefully become profitable or at least have an exit strategy.