Other systems like SQL server solve this with hints (HOLDLOCK) which is also discussed here that PG really needs hints.

So PG doesn't like hints and instead makes non standard syntax for upsert instead of standard merge syntax with a hint.

Now PG is adding merge anyway and probably could have just extended it with non standard syntax to get an atomic upsert instead it has two different approaches.

IMO it probably would have been better to go with the SQL standard MERGE with a non-standard hint syntax so its more readable between systems instead of adding non standard syntax then adding the standard much later.

Not sure if there is any movement on adding the upsert approach to the SQL standard instead.