Short answer: It’s a turn of phrase the Incel movement popularized as a dehumanizing dogwhistle.

Long answer: In current American English, “Female” and “Females” are highly formal terms and really are only commonly used in situations like law and academic discourse (this is true of “male” and “males” as well, though there’s much less cultural baggage associated with those terms). People who use them in casual conversation instead of the much more common “Women” (or the diminutive, “Girls”) tend to be the kind of person that uses formal language to emphasize their own intellectual superiority over the common masses, and in particular all the women who won’t have sex with them. In fact and almost invariably, this is presented in the form of explanations about why nobody wants to have sex with them, and the cultural forces that are causing it, and why deep down it’s the women’s fault they can’t get a date, and it just all goes downhill from there.

edit: clarity