This is a common thing in many languages.
We use base 10 now, and we count easily and in an organised way into the thousands, but historically many things were measured in other quantaties, like twelves or even twenties (score, see meaning 1.b at MW).
So before people ever had a necessity to count large numbers in a structured way, they would use words for quantities that occurred often in everyday life. Numbers up to a certain point are older than the structural way in which larger numbers are created.
In English, some regularity arises from thirteen on (three+ten), indicating that commonly people counted up to twelve (a dozen). In French, this goes up to 16 (seize).
Who decided that the number 10 could not just be prefixed with the next number, like it does from 20 - 99.
The same person that decides on every word that is used in a language... nobody.
The lower the number, the older the word likely is. In many languages, including English, the word for one is also used as a pronoun (English) or an article (French, Dutch). It's not hard to imagine that it is also the first number that got a word assigned to it.