(Or, for that matter, why not onety-one, onety-two?)
These are questions that kids acquiring English frequently ask, as Ruthie does in this One Big Happy strip:
(#1) Kids are pattern-seeking organisms, and when confronted with a deviation from a pattern, are inclined either to silently, unconsciously, eliminate the deviation in favor of the pattern; or to explicitly lodge a complaint, as Ruthie does here
Ruthie asks a why-question, and there is an answer to it — a complex, fascinating answer supplied briefly for the general reader by Arika Okrent in several places (see below) — but it’s of no use to Ruthie, because that answer is about the history of English, while she wants to know what sense it makes for English to work this way now, and why don’t we fix it.
Alas for Ruthie, the only fair answer to her question is that English just is as it is, and it doesn’t really make sense. Bite the bullet, kid.