I was referring to a single instance of classic PostgreSQL running on a single server. Scale-out databases are a completely different animal, it is difficult to compare them to scale-up databases because they make a different set of tradeoffs.

Extreme scale-up database engines are relatively rare and usually bespoke, I am not aware of anything close in open source. They only have one major advantage -- they are easily deployable outside the data center, e.g. at the edge, because it is a single self-contained box and relatively power efficient. Most people opt for scale-out because 1) there are many to choose from in open source and 2) it will work just as well as extreme scale-up for most applications, though you'll need more hardware for the same workload.

I've started to appreciate the utility of the extreme scale-up engines in real-world applications, though I've mostly worked on scale-out engines. They solve a real problem at the edge for data intensive applications but most currently people deploy in the data center so there isn't a pressing need.