Cultural perception: The more a field is perceived to be about "things" instead of "people", the more technical it is.

I posit that MechE, CS and physics (to an extent) are very much culturally perceived to be more about "things" and less about "people".

To me it is obvious that women in general to a greater extent than men want to do work with a more significant social component.

Hence, it follows naturally that the more specifically technical a field is perceived to be, the less likely women are to pursue a career in said field.

Other fields, like chemistry and mathematics have transferable knowledge to, for example, pharmacology and teaching, respectively. This isn't so much the case with the fields that still feature low engagement among women.

The people vs. things hypothesis has pretty robust data supporting it at this point.