I'm not the most ceremony oriented person in the world but. over 30% of american teenagers/young adults drop out of high school, when they already spent 10+ years in the education system, etc. This is not to mention how high school being open to everyone for free in America isn't something that's alawys been available.
I could go on to say why graduating from high school is important, but the point is school isn't cut for everyone, and for those who do do well in it, ceremonies are generally the only real way of making recognition about it all. I know that's idealistic, and yes you can always determine if you want to attend or not, but having the opportunity to be represented is well enough.
I don't, however, understand this brutal selfish attitude of "It's MY graduation, why should I attend a ceremony for anyone else if I don't wanna go?!" My friend was the first of his family to ever graduate from High school.. it was a big deal for all of them to see him walk down the aisle. It's a proud thing to see your child do that. It's not like something you did was SO important that day that you couldn't take 3 hours of your time to attend a ceremony.