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'.