Between plain old horizontal scaling via custom sharding, replicas, and extensions like Citus and Timescale that offer full horizontal scalability, Postgres is handling some of the biggest use cases in the world.
And it comes with a bunch of caveats. It's bolted-on horizontal scaling, vs MongoDB where that's the main feature. Yes I still prefer Postgres in general, but it's fair to point out that weakness of it.
Citus was acquired by Microsoft so there are doubts about its future longevity given that so many similar promises of ongoing support are rarely maintained. Timescale is optimised for time-series so not sure it's applicable.
Point still remains that PostgreSQL lacks a modern, built-in horizontal scalability solution.
Well let's see how long Google's Spanner lasts. It's designed to support huge-scale systems, and personally I'd be terrified locking a huge system into that.
Postgres does have things like Citus, or even wire-protocol compatible things like CockroachDB that do scale, but those are not Postgres.
It's the same situation with MySQL vs. Vitess.