Because they have been brainwashed by the R&A branding, or, are victims of British exceptionalism, which they feel means they shouldnt have to state 'British' at all.Why does it bother people SO much if someone calls it “The British open”?
Its a modern thing. A few decades ago, while 'The Open' might have been used as affectation, most every body said British Open, and nobody batted an eyelid.