systematic regularization

The reason, according to MIT linguist Steven Pinker in his 1994 book The Language Instinct, is systematic regularization.

It works like this. The rightmost component of complex and compound words almost always bear the semantic weight, or root meaning of the word. Linguists call this meaning-root the head of the word. Humans interpret the meanings of compound words per their head. For example, the head of overshoot is shoot, because it is a kind of shooting, not a kind of over-ing. Furthermore, it is a verb, because the head is a verb.

When we process such words, the rules for the head get percolated up to the entire thing. We will conjugate overshoot per the rules of shoot, to arrive at overshot, keeping with the irregular inflection shot instead of overgeneralizing past tense grammar to produce overshooted.

Most words work this way.

But there are certain words and word phrases that are Ichabod Craney, that is, they're headless. This occurs when the meaning has drifted far enough from the root that the rules of the root no longer apply. For instance, Mickey Mouse has no head, because a Mickey isn't a type of mouse like a church mouse, he is a particular mouse, and so obeys rules of a proper noun if he is ever pluralized. Thus: "I am so sick of the Mickey Mouses running this organization." Similarly, a saber-tooth is not a type of tooth, it's a type of cat, so prides of them are saber-tooths, not saber-teeth. More fun, falsely-pluralized headless words: low-lives, tenderfeet, flatfeet, still lives.

Similar rules and exceptions apply to drifted complex and compound verbs. To fly out in baseball is a verb phrase that is not a type of flying. Fly is a categorical adjective in this phrase, as in fly ball, but it still carries the root meaning as the verb. Its past tense inflection is therefore flied out and not flew out. Similarly we say someone grandstanded to the crowd, not grandstood.

This process of semantic drift from the root and resulting change of inflection is called systematic regularization.

This effect also explains why the "grammatically correct" attorneys general sounds so odd to us. Modern use of the phrase is headless, i.e. it has acquired the status of a single word. We don't ever need to distinguish the attorneys general from attorneys specific. It's now a thing unto itself and sounds right when it obeys standard inflection as attorney generals. The rules that insist on pluralization on the first word reflect an archaic usage when the second word was a distinguishing mark, and was probably emphasized in speech. Nowadays we use it like one word, and the "proper" pluralization is a little pompous if correct. Terms like this and sergeants major and brothers-in-law strike us as if we were hearing Mickeys Mouse.

If anyone ever corrects you on one of these words, defend yourself by pointing out that the term has been systematically regularized, and in their stunned confusion you can escape.