Then why is the USA the only country using indirect elections?
While I think the electoral college is pretty nutty, in defense of the USA, they're not alone in their use of indirect elections.
Virtually every jurisdiction using the Westminster Parliamentary System (mostly Commonwealth countries like the Canada, Australia, the UK etc.) use indirect elections.
A riding ('district') elects a Member of Parliament (MP) who heads off to the legislature. The party with the most number of MPs form government, and the leader of that party becomes Prime Minister. So in that sense, the PM is 'indirectly elected.'