"Those who forget history are doomed to repeat it" - George Santayana
"Woman" derives from the combination of the words wif (lit. woman) and man (lit. human being). "Man" , on the other hand, used to be "wer"*. Going to werman in the same pattern that took wif to wimman, then getting dropped just to "man".
Why? Most likely because, over time, the assumption that wer was the default sex/gender, that you only had to specify gender when talking about the other gender. This has actually already happened in Esperanto, a language only 100 years old (based on language forums- I don't know Esperanto myself)- so clearly whatever caused it to happen isn't out of our systems yet. The distance between "man", a gender non-specified term for people, ended up being uneven- reflecting the same idea that "generic he" carries.