humans can be sued. what about AI? or even a commercial software?
Not everywhere in the world do companies count as people, yet they can still be sued.
I'd wager the companies lobbied for this to gain extra rights.
Actually yes, everywhere in the world. That has a functioning legal system, at least.
If companies weren't treated as legal persons, they wouldn't be able to enter into contracts.
But also, just to be clear, a legal person, like a corporation, is not a natural person. Unlike a natural person, they can't vote. There isn't anywhere in the world that considers corporations to be natural persons.
EDIT: Note that ianal, nor a historian. The specifics of how this came about are best learned from a more authoritative source.
In theoretical sense sure.
In a practical sense? They are invulnerable due to what can be extreme financial obstacles they can put in place. They can drag a court case out until you fold if you haven't found a lawyer willing to do it on contigency.