Link: https://en.wikipedia.org/wiki/Copyright_Act_of_1976

Section 302 of the Act extended protection to "a term consisting of the life of the author and fifty years after the author's death". In addition, the Act created a static 75-year term (dated from the date of publication) for anonymous works, pseudonymous works, and works made for hire. Under section 304, the extension term for works copyrighted before 1978 that had not already entered the public domain was increased from 28 years to 47 years, giving a total term of 75 years.

...

According to section 408 of the Act, registration of a work with the Copyright Office is not a prerequisite for copyright protection. The Act does, however, allow for registration, and gives the Copyright Office the power to promulgate the necessary forms. Aside from Copyright Office paperwork, the Act requires only that one copy, or two copies if the work has been published, be deposited with the Office to accomplish registration. Though registration is not required for copyright protection to attach to a work, section 411 of the Act does require registration before a copyright infringement action by the creator of the work can proceed. Even if registration is denied, however, an infringement action can continue if the creator of the work joins the Copyright Office as a defendant, requiring the court to determine the copyrightability of the work before addressing the issue of infringement.

Sonny Bono served as a U.S. representative in the mid-90s and, for some reason, sponsored the copyright term extension act. I'm sure it was altruistic on his part. /s

https://en.wikipedia.org/wiki/Copyright_Term_Extension_Act

Since 1990, The Walt Disney Company had lobbied for copyright extension.[14][15] The legislation delayed the entry into the public domain of the earliest Mickey Mouse cartoons, leading detractors to the nickname "The Mickey Mouse Protection Act".[1]

In addition to Disney, California congresswoman Mary Bono (Sonny Bono's widow and Congressional successor), and the estate of composer George Gershwin supported the act. Mary Bono, speaking on the floor of the United States House of Representatives, said:

"Actually, Sonny wanted the term of copyright protection to last forever. I am informed by staff that such a change would violate the Constitution. ... As you know, there is also [then-MPAA president] Jack Valenti's proposal for term to last forever less one day. Perhaps the Committee may look at that next Congress."[16]

About old Jack, he had everyone's best interests at heart. Also /s.

Jack Joseph Valenti (September 5, 1921 – April 26, 2007) was an American political advisor and lobbyist who served as a Special Assistant to U.S. President Lyndon B. Johnson. He was also the longtime president of the Motion Picture Association of America. During his 38-year tenure in the MPAA, he created the MPAA film rating system, and was generally regarded as one of the most influential pro-copyright lobbyists in the world.