I was just shitposting while high. but the real question isn't "why are they pronounced differently?" but "why are they written the same?"

Woman is an ellison of wīfmann

Man is an abbreviation of wermann

"Mann" meaning "human" and wīf/wer meaning "female/male"

No one asks why "man and men" are pronounced differently, and it's likely we'd have "wermann / wermen" pronounced "wur-man" and "wier-men" if we'd kept the distinction.