PostgreSQL has unique indexes, ON CONFLICT, MERGE queries etc. It is not that easy to correctly implement that with ClickHouse (ReplacingMergeTree etc).
PostgreSQL has full text search capabilities and faceted navigation can be easily built on top of it.
PostgreSQL has lots of decent options for backups. ClickHouse recently got built-in backup capabilities too, but it was not available "back in the days".
When you run a cloud service, having multiple databases offers extra flexibility, for example, I can stop ClickHouse database for maintenance but PostgreSQL will be still available.
For practical reasons:
PostgreSQL has unique indexes, ON CONFLICT, MERGE queries etc. It is not that easy to correctly implement that with ClickHouse (ReplacingMergeTree etc).
PostgreSQL has full text search capabilities and faceted navigation can be easily built on top of it.
PostgreSQL has lots of decent options for backups. ClickHouse recently got built-in backup capabilities too, but it was not available "back in the days".
When you run a cloud service, having multiple databases offers extra flexibility, for example, I can stop ClickHouse database for maintenance but PostgreSQL will be still available.