> Clearly - if you are paying 10s of thousands of dollars for a database server License, it makes sense to fully utilize the ones you've purchased.
I suspect this is the reason they're using a single server. It would cost a few more tens of thousands of dollars to install another one, versus the zero overhead of just upgrading the existing one.
As an OSS user, I often take for granted how easily I can go "Oh, the database reads are too slow? Let's throw another Postgres slave at it and see how that goes." After all, that expense is usually minimal.