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.
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.
sure, citus is an extension of postgres and part of ecosystem.
Point still remains that PostgreSQL lacks a modern, built-in horizontal scalability solution.
Microsoft on the other hand is among the best in keeping such promises.
If Google had bought it, sure...