It’s very rare that you want a web client to maintain an open connection for a long time. It would be a scaling nightmare.

I’m also trying to imagine validation. Parsing SQL error strings? Kill me now.

PostgreSQL connections don't scale up as well as some other databases. Every connection gets a huge chunk of server memory so you'll run out of memory very fast.

You don't want to give the internet at large unfettered access to your database.

Even if you have row-based security.

I often do it with ClickHouse, which has HTTP API included in the database server.

For example, https://play.clickhouse.com/ is an open public playground, and https://adsb.exposed/ is a full-featured application with HTML+ClickHouse and no dedicated backend.