Ask a question in a lecture? The 100+ other people in the room are now staring at you like you're an idiot, despite the fact that most probably have the same question.
Ask for clarification on something the lecturer said? You've interrupted their flow now, and want you to sit down, shut up and ask after class.
Go to ask after class? 12 other people have questions they were too afraid to ask during class or couldn't and the lecturer doesn't have time to attend to all of you, and tells you to send them an email they won't read.
Ask a TA? Most of them are overworked and underappreciated which leads to them having a very low tolerance for "I don't understand" questions, which can lead to a brush-off answer like "just re-read the lecture material."
Ask other students? They either don't know themselves or cannot explain it in an appropriate or correct manner, or simply cannot be bothered to help you because "this is too easy, why don't you get it!?"
Now of course, not every lecturer/TA is like this, and I had some wonderfully helpful and open teachers while I was in university, however, both myself and other people I know from across many different schools have had numerous experiences like this.
And here's the other thing, as a student I've been guilty of doing the exact things I describe above, either out of arrogance or simply down to the fact that I was slammed with other classes I just didn't have the time or patience to help others. Is it something I am proud of? Of course not, looking back now I can safely say I was a dick.
It gets even worse for women, when you differ from the majority of others in your class simply by genetics, it creates a feeling of not wanting to stand out or give other people a reason on top of their sub-conscious bias to think you "don't belong here" or "aren't as good as the rest of us."