This! You NEEDED to scale horizontally because machines were just doing too much. I remember when our Apache boxes couldn’t even cope doing SSL so we had a hardware box doing it on ingress!
I proposed dumping them all for a smaller set of x86 hosts running linux, it took 2-3 years before the old admins believed in the performance and cost savings. They refused to believe it would even work.
To this day I still argue with ex Solaris sysadmins.
Those days, you had to carefully architect your infrastructure and design your workload to deal with it, and every hardware improvement required you to reevaluate what you were doing. Hence novel architectural choices.
Everything is way easier for normal sized organizations now, and that level of optimization is just no longer required outside of companies doing huge scale.
It was never about the actual workloads, much more about growth projections. And a whole lot of cargo cult behavior.