Super biased, layman thoughts incoming.

Re. countries vs corporations: countries in isolation make laws for themselves (consumer protection, long-term economic protection, etc.), whereas corporations must abide by the laws of their host countries (hence tax evasion via incorporation in accommodating countries). If two countries start to compete, there isn't really anything that stops them from annihilating one another in disturbing fashions (e.g. erecting walls, slinging propaganda, blockading, etc.) other than treaties. Get enough treaties together and you get the EU.