We treat corporate speech as an extension of human speech because corporations are made up of humans, and there is no way of abridging the speech of corporations without also abridging the speech of the humans within that corporation.
Also, abridging corporate speech creates a precedent for abridging the speech of all human collective entities like religions, political parties, advocacy groups and the press, because there is no particular reason why corporations as an abstract entity created by humans to express collective human will should be unique in this regard.
You can't simply nullify rights when people exercise them in aggregate. That isn't how free societies are supposed to work, and it's too easy to undermine.