While "onety-one" or "oneteen" would seem to make sense as pronunciations for the number 11, the origin of the word "eleven" is the Old English word "endleofan," literally "one left" (over 10). How words are pronounced as of 2014 is influenced by how they were pronounced in the past.
Counting items by tens is still a common practice, as it's easy to organize a decimal scale into tens. Someone who had 11 of something would think of it as 10 plus one, or one left over 10. The word for 12 follows this same format and comes from the Old English "twelf," literally "two left" (over 10).