Why are women even less represented in engineering than in other STEM?
I suspect that this is yet another example of the fractal nature of the gender binary. STEM may be coded masculine in general, but within STEM different fields are coded "more masculine" than others.

I'm very familiar with this in my own field: IT in general is masculine, but UI/frontend is more feminine than backend, on the backend system stuff is more masculine than application stuff, writing it in C is more masculine than writing it in ruby, ... This is sometimes even explicitly expressed as "your beard has to be this long to write/understand this code"

I've never found a good explanation for most of these associations, they seem fairly random to me (and apparently change between cultures as well).