[Hansuke]

We all know that Onyomi is the Chinese reading of a character and Kunyomi is the Japanese one. I also know that Kunyomi is used when a Kanji is alone or with hiragana attached to it. Why, then, we read the numbers using Onyomi although the Kanji is written alone?
一 is pronounced いち and not ひと (I believe this is its Kunyomi)
I wonder why!
I would really appreciate any help.