The Postgres team decided against implementing MERGE when they designed the ON CONFLICT feature (it actually started as a MERGE). It's been a while, but as far

The Postgres team decided against implementing MERGE when they designed the ON CONFLICT feature (it actually started as a MERGE). It's been a while, but as far as I remember, the main reason was that the behaviour of MERGE mandated by the SQL standard, wasn't clear enough and doesn't deal with race conditions when multiple transactions try the same insert (or update) (not so much the syntax itself).