It wasn't. The misconception that SCOTUS declared "corporations are people" is a meme launched by those who think people lose their rights when they decide to act together. SCOTUS just confirmed they do not. Corporations are not people, never been, never will be, and you don't need an act of Congress to know that - you just need to be a bit better informed.
There is a legal concept of "corporate personhood", but it has nothing to do with "being people", it's a legal term of art. For example, it makes it possible to sue a corporation instead of trying to sue each of its shareholders and executives individually. And it wasn't used in Citizens United v. FEC at all.
There is a legal concept of "corporate personhood", but it has nothing to do with "being people", it's a legal term of art. For example, it makes it possible to sue a corporation instead of trying to sue each of its shareholders and executives individually. And it wasn't used in Citizens United v. FEC at all.