why diploma is so important?

Because you have no proof that you learned any skills from school (not that anyone learns any life skills from high school classes, but still...). I bet you could help yourself a ton by taking the GED test.

While I agree with you that a person's abilities are not determined by the diplomas he/she holds, there are good reasons why corporations want you to have a high school diploma. They need official proof that you can actually do the work you claimed to be able to do. They can't just believe what you say, and that piece of paper is the proof. Certainly there are many people whose abilities go beyond that piece of paper--they may have learned things on their own that the paper does not cover. But it would do you some good to get that paper if you're hoping to find a job soon.

Still, having a high school diploma serves as proof of your hard work and all the trials you've experienced in the high school life. Even though your a self-studier, you still need a proof for you to prove to other people that you really have graduated from high school. It also serves as a remembrance for other people- they'll just look at their diplomas and they'll remember their days when they were still in high school. The sufferings they endured. The hardwork they did that finally paid off. The friendship shared between acquaintances. The trials they bravely conquered and also the happy times they shared with each other when they were in high school. The diploma not only serves as a proof of your high school education but also a treasure to many people.