Back in the day, hahaha, when I was in school, there where only 2 graduations by the time you finished 12th grade. Kindergarten and high school.
Many kids go to Pre-School now and have a graduation before they even go to Kindergarten. Complete with caps and gowns.
I think kindergarten graduation started because kindergartens were privately owned when they first started. When public schools started offering kindergarten, they kept the graduation. That’s my theory, anyway.