There are some historical reasons, in short:
一: one bar. May mean the whole, the universe. Reference.
二: two bars. May mean the positive and negative. The upper may refer to the heaven, the lower the earth. Reference.
三: three bars. The upper may refer to the heaven, the lower the earth, and the middle the human. Reference.
四: First |||| (four bars), then combines with 二 to avoid ambiguity: , like being mistaken to be two ||. Reference.
- 五: First a 二 with X in between: . The two bars are sky and earth, the X being the intersection between them, to mean all positive number greater than four. Reference.
- 六: ￼, a house with four sides plus roof and ground giving six. Reference.
- 七: Designed originally to be 十 (meaning all number greater than six) with a twist on the bottom: . Reference.
- 八: Opposite bracket-like curve meaning "divide", used to be greatest number. See 八卦. Reference.
- 九: Designed originally to be a right arm () reaching for something. Nine bears a meaning of great, the number of categories for many things. Reference.
- 十: | (a rope overhung) combines with a knot on it: . Using ten to mean "all" may be because of the fact that we have ten fingers. Reference.
Languages are not designed by one inventor once and for all.
There is an evolution. At first, people invent those most frequently used: one, two, and three, as put by 老子 in 「道德經」: 道立于一, 一生二, 二生三, 三生萬物. ("The Tao produced One; One produced Two; Two produced Three;
Three produced All things." translated by James Legge ) Later, patches have been made to enlarge the characters set, based on growing demand that just using "All thing" for large amount of items is not enough for everyday usage. A lot of exceptions are used to create new words, just like "September" means the seventh month before.