Translation:They are women.
If anything the question should be why isn't it "אישות" because the singular is "אישה".
There's an old Aramaic word for man that made it into Hebrew: "נש". Although it is masculine, this is used mostly as a gender-neutral person. And yet נשים is obviously derived from that while still being the plural of אישה.
So the answer to your "why" question is "because languages are weird"
Are you sure? This is also the suggested translation. What was your error message?