I never knew the Oxbridge unis did that with MAs - that's ridiculous, unlike everything else associated with those establishments.
“Contrariwise,' said Tweedledee...
You have to realise that Oxford and Cambridge were much earlier than other English and US universities (by centuries). Their MA wasn't an advanced degree. it was a sign of post-degree experience and maturity, being awarded to BAs not less than 6 years after the end of their first term in residence, the recipient then becoming a member of the University Senate. It's these new-fangled establishments who have shown no originality in re-using it for an award for advanced studies.