There's a simple reason - it costs a lot of money to use it.
That's right, the song that Guinness World Records in 1998 called the most recognized in the English language is under copyright. It's owned by Warner Music Group, and for decades they have charged a license fee for the use of the song and rake in around $2 million a year in fees.
Movies and TV shows that feature a birthday party get around the fee by never completing the song (someone will come in and interrupt the singing), by singing "For He's A Jolly Good Fellow" (which is in the public domain), or by just not singing at all when it's cake time.
But some filmmakers do pay the fee because they feel it's important for the song to be featured. For the classic 1994 documentary "Hoop Dreams," filmmaker Steve James said he paid $5,000 for use of the song in a scene where one of his main characters celebrates his 18th birthday. It was a major feat for the film's subject, as he and his family lived in a section of Chicago with a high murder rate. The total budget for the film was around $700,000.
Vimeo/Fine Line/"Hoop Dreams"
It cost $5,000 to legally have the "Happy Birthday" song featured in "Hoop Dreams."
The melody for "Happy Birthday" goes back to 1893, when sisters Patty and Mildred J. Hill introduced the song "Good Morning to All" to Patty's kindergarten class in Kentucky.
But this is when things get interesting.
As the "Good Morning to All" songs began getting passed around from school to school at some point - there's no definitive date or who was responsible - the lyrics "Happy Birthday to You" appeared. However, the melody stayed the same.
In 1935 a piano arrangement of the melody was copyrighted by a company later known as Birch Tree Group.
In 1988, Warner Music Group acquired the song from Birch Tree for around $25 million.
However, a filmmaker recently brought a lawsuit in a New York court that may change everything.
Jennifer Nelson has been making a documentary about the "Happy Birthday" song since 2013. To her surprise the song wasn't free, and to use it legally Warner was going to charge her $1,500.
This led her to file a lawsuit in June 2013 seeking the court to declare that the song is in the public domain.
Meaning free for anyone to use for any purpose.
Nelson's company, Good Morning to You Productions Corp, claimed:
"Irrefutable documentary evidence, some dating to 1893, shows that the copyright to 'Happy Birthday to You,' if there ever was a valid copyright to any part of the song, expired no later than 1921."
It went on to state that if Warner has any claim to the song it's specifically to the piano arrangement it bought from Birch Tree in 1935.
"Significantly, no court has ever adjudicated the validity or scope of the defendant's claimed interest in 'Happy Birthday to You,' nor in the song's melody or lyrics, which are themselves independent works," reads the claim.
Business Insider reached out to Nelson (who is currently shopping around her film for distribution) on Tuesday to get an update on the lawsuit. Through her lawyer, Mark C. Rifkin, she states that a decision to determine whether "the copyright protects just the piano arrangement rather than the song itself" could be made by the judge as early as this summer.