For the vast majority of simple use cases the common subset of all popular SQLs is exactly the same. Otherwise… just use Postgres

It's not that different. I'd rather have a different way to do UPSERTs or a different window function here and there [1] than figure out every ORM's join syntax or its sneaky ways to SELECT N+1 me into oblivion.

[1] LLMs make these very easy to handle.

I would argue that is a bit like complaining there is no "backend language" and that Java, Rust, Go all have different syntax.

The choice of DB is arguably more important than the choice of backend language.