我看了下知乎其他人的回答，没有一个人的回答是有书面和历史根据的，有记载根据的是（节选自Mother Tongue -- Bill Bryson）：Until the eighteenth century it was correct to say ‘you was’ if you were referring to one person. It sounds odd today, but the logic is impeccable. Was is a singular verb and were a plural one. Why should you take a plural verb when the sense is clearly singular? The answer - surprise, surprise - is that Robert Lowth didn’t like it. “I’m hurrying, are I not?” is hopelessly ungrammatical, but “I’m hurrying, aren’t I” - merely a contraction of the same words - is perfect English. Many is almost a plural (as in “Many people were there”), but not when it is followed by a, as in “Many a man was there”. There’s no inherent reason why these things should be so. They are not defensible in terms of grammar. They are because they are.

看中文即可：上面这段我的翻译：(节选自比尔·布莱森的《母语》/Mother Tongue）：直到18世纪，如果你指的是一个人 one person，那么用“you was”是对的。今天这么用听起来很奇怪，但是这么用逻辑上是对的。Was 是动词单数形式，Were是动词的复数形式。为什么你要在表达单数意思时，用动词的复数形式呢？令人很惊讶，答案是 罗伯特·罗斯 (Robert Lowth)不喜欢这么用（译者注：you was）。“I’m hurrying, are I not?” 绝对是语法错误的，错得很离谱，但是 “I’m hurrying, aren't I” ——仅仅是相同词组的缩写形式而已（译者注：are I not 缩写成 aren’t I）—— 就是完美的英语。Many 是复数（比如“Many people were there”），但是当“Many”后面跟着一个 “a”时，比如“Many a man was there”。为什么这些事情（译者注：即这里提到的语法现象：1. you were 2. aren't I 3. many a man has）是这样的，并没有内在的原因。在语法范围内，这些事情是没法自我辩护的（译者注：即无法自圆其说）。他们是这样，因为他们就是这样。

译者总结：关于提问人的问题：“为什么 I am 的反义疑问句用 aren't I？”，根据英国英语权威比尔·布莱森的书籍《母语》/Mother Tongue中的记载，可能是很早以前，几百年之前，有某Robert Lowth 之类的英语语言权威人士，单纯地不喜欢某个用法，就把这个用法改成了另外一个用法；单纯地从英语语法上，和英语语言逻辑上，并没有什么说得通的解释。

罗伯特·罗斯（Robert Lowth）其人：（同样出自比尔·布莱森地《母语》）：Consider the curiously persistent notion that sentences should not end with a preposition. The source of this stricture, and several other equally dubious ones, was one Robert Lowth, an eighteenth-century clergyman and amateur grammarian whose A Short Introduction to English, published in 1762, enjoyed a long and distressingly influential life both in his native England and abroad. It is to Lowth we can trace many a pedant’s most treasured notions: the belief that you must say different from different to or different than, the idea that two negatives make a positive, the rule that you must not say ‘the heaviest of the two objects’, but rather ‘the heavier’, the distinction between shall and will, and the clearly nonsensical belief that between can apply only to two things and among to more than two.

最后总结回答题主的问题：你这个问题 aren't I，就是这样用的，就是历史上某个英国语言学家突然受了刺激，大脑一热，或者是个人爱好，然后出了一本书，然后规定所有英国人都这么用这个语法，大家一直遵守这个语法规则到了今天。

我个人认为，不是每个语法现象，追本溯源都是追溯到某个人地个人行为与偏好，有些从语言学角度上来是“进化出来的”，而不是历史上某个人定下的规定。