Because corporations are just groups of people operating as one legal entity as is required for many ventures which would be too expensive or risky for one person to take on by themselves. Do you think corporations like the New York Times should not have freedom of speech? Do you think the government should be able to arbitrary power against corporations like Planned Parenthood? Should governments have arbitrary power to limit what information corporations like your local internet service provider can transmit? Should whatever political party who is currently in power be able to steal all the money from the opposing parties corporate entity?
Its good to keep in mind corporations include a large class of entities like public colleges, newspapers, internet providers, charities, political parties, organized religions, public interest groups, and of course for profit businesses. Saying all these entities should have no rights is advocating for the state to have unchecked and arbitrary power over all of them.