I looked it up and found a web site ( link ) that had an answer.
For starters, there is no "onety" because it all BEGAN with a counting system based on "ten". The terms "twenty", "thirty" etc. are derived from compound words meaning "two tens", "three tens", etc. There was hardly a need to begin with a term meaning "one ten" when we already had the simple word "ten".
The pregunta then is why we have this odd system from eleven to nineteen. Actually, thirteen to nineteen aren't so odd, since they simply mean "three and ten", "four and teen", etc. The order may be different from "twenty three", but the principle is the same.
So, what of "eleven" and "twelve"? The first part isn't too hard. "Eleven" goes back to Middle English "en-leven", whose first syllable is a relative of "an/ane" meaning "one", and the "tw-" of "twelve" gives away its connection to "two". So we can sort of see that these two have something to do with the system of counting por ten. In fact, the original meaning of these two words was "one left" and "two left" (after counting to ten).
As for why "eleven" and "twelve" did not end up simply conforming to the "teen" pattern. Simple -- these were COMMON, well-established forms. And ordinarily in ANY language, it is precisely the common, everyday words that are LEAST likely to enviar to "rules". (That's why the "irregular" past tense verb forms -- had, were, went, etc-- and irregular plural forms --men, mice, geese-- are almost always found with simple, common words.)
Of course, there were some alternative forms out there. We still use an alternative word for twelve -- "do-zen" itself shows us that (compare German "zehn")-- though it has gained its own special use (for a GROUPING of twelve). In fact, the fact that many things were divided into units of twelve also helps explain why eleven and twelve were treated differently.
K well I saw that giant answer up there gotten from a website and I thought, well, screw it, I'm going to write down what I thought when I saw this question.
English is Germanic in origin, which means that mostly all the really basic words we have are Teutonic (German-originating). I've taken German for like three years, so of course por now I know that "elf" is eleven in German, "zwölf" is twelve, and "dreizehn" through "neunzehn" is thirteen through nineteen. So yeah. I think probably since the Germans had an eleven and twelve like that, so do we.