Post your question on r/legaladviceofftopic I bet you'll get some very interesting answers...
There are things that you do and a corporation do that a river can't do, and to describe this set of things laws use the term "person" this makes sense but the name is terrible. In the US, for instance, you can't sue a mountain if you fall and hurt yourself - you can sue a mining company that left equipment there, or maybe the national parks service for having unsafe trails or maybe even your friend Fred (i.e. Purdue Pharma) for pushing you off the edge of the cliff (maybe, more accurately, for failing to inform you of a dizziness side effect in a medication they sold you that caused you to fall) but you can't sue the mountain itself and, likewise, there is no way a mountain could be punished that we can comprehend - again, maybe if this mountain is owned by someone then, as a result of this suit, you are awarded partial mining rights or ownership of the mountain - but that is a compensation granted at the loss of some other entity, maybe a mining company, or the property owner or even the US government.
Either way, corporations and human beings have some legal commonalities that aren't shared with mountains.
elliekelly as you're actually a lawyer, can you express this better or clarify it?
To your point, corporations have always had "rights." The government can't just show up to your house and take your property without due process and, because corporations are "legal persons," the government can't just show up to company headquarters and take corporate property either. Corporations need some of the legal rights we grant to natural persons or else the corporate entity is pointless. But corporations also can't have the same rights as natural persons or else every stock broker is also a slave trader.
The reason the Citizens United ruling was such a big deal was because the court extended first amendment rights to corporations when those rights had historically only been given to natural persons. If you get a chance you should read the opinion because the rationale seems much more reasonable on paper than it is in practice. Contrary to public opinion, the court didn't rule that "corporations are people" (technically they always were) but rather that corporations are associations of natural persons and since natural persons can form associations to exercise their first amendment rights then they can form corporations to exercise their first amendment rights.
But for an entity whose purpose is to exist somewhere between human (definitely a person) and mountain (definitely not a person) it doesn't make sense to extend all of the rights of personhood to that entity just as it doesn't make sense to treat the entity the same way we treat a mountain. Lawyers can form associations to practice law but we certainly wouldn't allow a group to incorporate and have the entity sit for the bar exam... right? Citizens United certainly muddied the water in where the law draws that line.