The answer to Is a thumb also a finger? here on English stackexchange never addressed the "why".
Why is it that thumbs aren't generally considered fingers? Thumbs have one joint less than the "true fingers", sure, but as do big toes to other toes, and they're still "toes." Thumbs are opposite to other digits, sure, but if that's the deciding criterion, wouldn't the phrase "opposable thumb" a little redundant? Not to mention, "opposability" seems to be on a spectrum, from "non-opposable" to "peudo-opposable" to "opposable", and they're still called "thumbs."