The RETURNING semantics produce content of the resulting row after insertion. For the ones that have been skipped, this could be ill-defined. The content is not just the content of the row data that was supplied, but potentially server-generated content from sequences, row-level triggers, etc.

There's one HUGE upside for doing it the current way: autogenerated columns.

E.g.

    CREATE TABLE t (id SERIAL PRIMARY KEY, x INTEGER UNIQUE);

    INSERT INTO t (x) VALUES (42), (43) ON CONFLICT DO NOTHING RETURNING *;

This does EXACTLY the most useful thing if one of '42' or '43' conflicts.