So, this is both fascinating and remarkably simple once you learn some basics about how spoken language evolves.

Let's start here: say "woman" out loud. Now, say it again, and this time pay attention to your tongue. When you said "woman," did you pronounce the "o" sound at the front of your mouth, higher in your mouth, and the "a" at the back, lower in your mouth?

Now try this. Say "women," again paying close attention to where the vowel sounds come from in your mouth. First pronounce "women" as it is written- kind of like "woah men." Do you feel how much more work that requires that pronouncing "woman" does? Now, pronounce "women" as you naturally do. Assuming you are in North America, this probably sounds a bit like "wimmin."

Probably, this "wimmin" pronunciation feels easier and more natural. This is largely because those vowel sounds originate in roughly the same area. When a word has multiple vowel sounds and they move from front to back or top to bottom, there's a good chance we will naturally shift towards an easier pronunciation.