I’ll add context that NUMA machines with high CPU’s and RAM used to cost six to seven digits. Some setups were eight figures. They had proprietary software, too.
People came up with horizontal scaling across COTS hardware, often called Beowulf clusters, to have more computing for cheaper. They’d run UNIX or Linux with a growing collection of open-source tools. They’d be able to get the most out of their compute by customizing it to their needs.
So, vertical scaling being exorbitantly expensive and less flexible at the time, too.
People came up with horizontal scaling across COTS hardware, often called Beowulf clusters, to have more computing for cheaper. They’d run UNIX or Linux with a growing collection of open-source tools. They’d be able to get the most out of their compute by customizing it to their needs.
So, vertical scaling being exorbitantly expensive and less flexible at the time, too.