It's because it has nothing to do with intelligence, and everything to do with presentation. This is why some people work in sales, and some people work in engineering. If someone that has no clue keeps insisting they do understand, at some point you'll start doubting yourself instead.
Richard Feynman, who is without much debate one of the most important physicists of all time, said something about asking "dumb" questions, and how people often misunderstood why he was asking the things he was asking. He was trying to look at something from different angles, but if you can't look beyond the question, you might think "why is this person asking me something a child would ask". They're not stupid, they're trying to initiate an exploration of ideas.
In my opinion, when you ask a "dumb" question to someone who is more concerned with looking smart and presenting well, their first reaction will be one of confusion, or even mockery. When you ask a truly intelligent person a "dumb" question, they will humor you, and start looking at the problem from different angles, usually trying to involve you in the process. Smart people love to discuss things, "presentable" people hate it, because for them it's just one long continuous risk of being "exposed". Smart people do not fear being exposed, because they know very well what they don't know, which is one of the most common traits among intelligent people, and ironically is why intelligent people feel dumb.
One of the most important lessons in my professional life is perhaps similar to yours, and that is that it's easy to recognize true intelligence when you encounter it. Truly intelligent people are normally humble, curious and intense. When you encounter people that are arrogant, narrow-minded and lazy, you're not the one in the wrong. They are just hiding their insecurity.
So to the point of the post, what should you do in a situation like this? If you're extremely introverted and not used to working in teams, that is a huge hurdle, because that is how human beings normally seek assurance: through their peers. So if I could give only one general advice here, it would be: work on your people skills. Try to pair up with colleagues, discuss their work, your own work, and be humble. Be open about your doubts, and you'll quickly realize that there is no line of business more full of people with impostor syndrome than IT.