It probably IS expensive to host because it’s written in a language no one uses without an optimizing compiler, and apprently most of the “db” is flatfiles on disk, some of which get read on every request.
Computers are fast. This isn't a particularly demanding workload; pages for users that aren't logged in are cached, and there probably aren't a ton of people logged in.
Last time I looked, Let's Encrypt had one big beefy Postgres database to issue pretty much every single TLS certificate on the Internet. Computers are fast.
