Women also make up a low percentage of janitors, coal minors, construction workers, and landscapers.
Granted it's usually less labor intensive than some custodian duties.
As far as construction, land scaling and mining well for the most part it's due to sexual dimorphism.
The average women has considerably lower upper body strength than they average man.
Jobs that involved throwing 60lbs bags of cement or fertilizer over ones shoulder aren't going to bode well for most women.
If a women isn't a CEO or engineer, we assume women are unfairly being discriminated against. If a women isn't a low value job, we assume they aren't biologically ideal for it.
That said it doesn't mean there is no discrimination either.