>> Visas are overwhelmingly only granted to technical fields. So people who can immigrate to america are the ones in technical fields.
Even so, why are there more of those technical immigrants female than the locals? That the majority of visas granted are for technical fields doesn't tell us anything about the gender of those who obtain them. There's nothing stopping 100% of immigrant IT workers from, e.g. India from being male. Why are they not?
Also, I'm not sure the assumption that visas are only granted to technical workers is correct. My experience is from the UK and I don't know what happens in the US, but over here, immigrants do all sorts of jobs. For example, many doctors and nurses in the NHS (the National Health Service) are from India or the EU; the majority of agricultural workers are from the EU; plumbers, famously, are all Polish; catering workers are from the EU; etc etc.
> Even so, why are there more of those technical immigrants female than the locals?
You are comparing locals( general population) to immigrant females( self selected group). Ofcourse population selected for technical degrees has higher % of technical degrees.
> There's nothing stopping 100% of immigrant IT workers from, e.g. India from being male. Why are they not?
Because all people want better life male or female. Getting into tech(even if goes against their inherent choices) for Immigrant female population is worth the high reward vs local population where upside of getting into tech is might not be worth going against their inherent interests.
> the majority of agricultural workers are from the EU
You explained it yourself. What you are asking is equivalent to 'why aren't there more of UK natives among agricultural workers'.
You won't see lots of indian students studying philosophy because once you are done you have to go back. Bad monetary investment.
Want a better life = study engineering. The explanation is as simple as that.