Actually, corporate personhood dates back farther than that.
In
Dartmouth College v. Woodward, 17 U.S. 518 (1819), the State of New Hampshire tried to force the trustees of the College to reinstate an ousted President. The Supreme Court held that a corporation was entitled to constitutional protection because the owners of the corporation were themselves entitled to constitutional protection, and the corporation is merely a manifestation of their collective right to contract under the Contract clause of the Constitution.
This decision was based on an earlier decision --
Fletcher v. Peck, 10 U.S. 87 (1810). There, the State of Georgia had sold public lands to private individuals by a public act. It was later revealed that the Act was procured through bribery. The State of Georgia repealed the Act and then tried to repossess the land that had been sold. However, some people had already sold the land acquired through the Act to innocent purchasers. The Supreme Court found that the Contract Clause barred the States from extinguishing contract rights granted to its citizens.
Fletcher was one of the first Supreme Court decisions after
Marbury v. Madison (1803), which established the Supreme Court's right to invalidate unconstitutional statutes.
So it is incorrect to think the Supreme Court came up with this doctrine of recognizing corporations as individuals 100 years after the fact. Granting corporations rights is a natural outgrowth of the right to contract, and has been recognized since the birth of this Republic. Corporations are the manifestation of the will of the individuals who comprise the owners of the corporation, and thus retain the rights of the individual owners. The only issue has been whether there are any individual rights a corporation does not possess.