Or that there's much, much more to the social and economic makeup of the country than who's objectively the best at something. If that was how it is, and every person received an equal education, then yes, it would be insulting to think that people are not all on equal footing just because one didn't pull themselves up far enough. However, that's not the case in most places. Jobs are decided by a hair at times with so many people needing them, and even people that don't actively hate women can subconsciously think that a man just "looks" stronger and therefore is more likely to close a deal or get the client or whatever. Enough people thinking that and you've got yourself a male-dominated industry, not because women are less talented but because on equal footing in everything except gender, people would be more likely to pick the male over the female.
It's not a matter of people that actively hate women. Although I'm sure those people still exist, they're not common anymore and aren't a real issue in the general workplace. Instead it's about the subconscious judgment.
Your statement "You are prejudging them as being unable to attain things for themselves" is true, but not in the way you meant it. It's not a judgment that they're not talented enough or smart enough or good enough to get what they want - it's a judgment that society perceives different genders as inherently "better" at certain jobs, and therefore pressures out women that want to work in that field and chooses men over women with equal qualifications. Women are less able to attain many things, no matter how hard they work.