Because it's a fully featured public relations platform, not just a "newsdesk" (though that's what it started as some 20 years ago).
We have a main monolithic application at the core. But there are plenty of ancillary applications used to run the various parts of our application (eg. analytics, media monitoring, social media monitoring, journalist databases, media delivery, LLM based content sugestion etc).
Then we have at least one staging deployment for each app (the monolith has multiple). All permutations of apps and environments reach about 50 applications deployed on the platform, all with their own highly available databases (Postgres, Redis, ElasticSearch and soon ClickHouse).
You don't need anything. You choose the most convenient tool according to your professional judgment. I certainly hope that nobody is using Kubernetes because they are against the wall, and instead decide to use it for its features.
From the documentation on the site it says that they're running on dedicated servers from Hetzner... So they aren't auto-scaling anything, they are paying for that hardware 24/7. It makes absolutely no difference what the number of running containers are, the cost remains constant.
Ruby On Rails is well known for not being at the fast end of the spectrum, so it needs lots of machines, and lots of machines gives reason to user Kubernetes.
A NewsDesk application written in something compiled for example golang would be much faster and likely could run on a single server.
The benefit of single server being you don't need kubernetes and can spend that development resource on developing application features.
In theory, from a performance point of view, we could easily run our main Rails monolith on a single server.
One does not chose single server deployments when reaching for four nines of uptime though. We also run a lot of ancillary applications / staging environments etc which is what warrants a cohesive deployment platform.
No doubt with Github, AirBnB, Shopify and other big sites RoR is bigger for the front end.
But now if lots of those sites are running on K8s with Argo CD or something or on a cloud platform where the infrastructure is provisioned with Terraform Go is supporting a great deal of things but it's far less visible.
Wouldn't a single machine and a backup machine do the job?