Avoiding distributed systems problems. Distributed systems are so incredibly hard to get right that I will vertically scale postgres until I hit an insurmountable wall before giving in.
You can also build distributed DBs with PG. For example for a DB with multiple write nodes all you need to do is implement an event shipping model with logical replication where your servers publish their events and subscribe to others, and you need to implement conflict resolution rules, naturally. I think PG's type system can probably be leveraged to make a CRDT system on top (and I bet someone's already done it).
IBM mainframes were created for you. Imagine you had a single computer that had multiple nines reliability. Hot swappable disk, RAM, CPU. Redundant power supply. Redundant network stack. OS designed to never need restarting. That's basically what a mainframe is, and IBM sells billions of dollars worth of them to this day.