I'll make an excuse for PostgreSQL lacking that feature: it's a transactional relational database with extremely well understood performance characteristics and a reputation for robustly delivering new reliable features without breaking backwards compatibility.
Adding sharding-style horizontal scaling to that while maintaining the characteristics of PostgreSQL that make it so popular is a very tall order. My suspicion is that handling sharded horizontal write scalability through external systems and extensions is a better architectural fit than trying to bake that into core.
They have added horizontal scalability in the form of built-in replication: https://www.postgresql.org/docs/current/high-availability.ht...
Adding sharding-style horizontal scaling to that while maintaining the characteristics of PostgreSQL that make it so popular is a very tall order. My suspicion is that handling sharded horizontal write scalability through external systems and extensions is a better architectural fit than trying to bake that into core.