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.

The trouble is actually breaking that premise. The SC has ruled, effectively that money is analogous to speech. Undoing that would pretty much need an amendment to overcome. They could also find that trying to limit corporations at this point could be a violation of the 14th amendment.

I don't like it, I really don't agree with it, but the reality is that's what it would likely take to overcome counter arguments in favor of continuing corporate personhood. As I mentioned, it might be tolerated if it came with tax exemption, in leiu of new taxes against corporate/stock loans and more taxation on profit distribution.

It's not a problem because corporations never had "the rights of people" - people still do. People have free speech rights, and people can go to jail for crimes.

I'm not sure what you mean, people working for companies can and have been jailed. Or did you mean the legal entitiy of a corporation can't be 'jailed'?

Like Unions being forbidden to strike by courts?