The Guinness Book of World Records said that “Happy Birthday to You” is the most recognized English-language song. There’s no reason at all to doubt that claim. While “Somewhere Over the Rainbow” may be more beloved, there are fewer occasions to sing that song, and it’s a lot harder to sing it well.
In other words, the song “Happy Birthday to You” is part of everyday life. It would be surprising if friends and families celebrating a birthday didn’t sing it. It’s like the National Anthem before a ballgame, like saying “I love you” on Valentine’s Day, and like saying “good morning” in the morning.