Why do people think I’m stupid?

Being treated as less capable than you are is painful, and it is rarely about your actual intelligence. People judge competence quickly from surface signals, then read everything you do through that first judgment.

What’s usually going on

Those surface signals include how quickly you answer, how sure you sound, how you handle not knowing something, and things that have nothing to do with ability, such as age, accent or being new. A thoughtful pause can be read as confusion. A careful question can be read as not understanding.

Once someone has formed that view, they notice what fits it and overlook what doesn’t. The same mistake that would be forgiven in someone they rate highly becomes evidence against you.

It also shapes how you behave around them. Feeling underestimated, people often hold back or over-explain, which can confirm the very impression they’re fighting.

What you can do

Show your reasoning, not only your answer. People trust conclusions they can follow.
Prepare one strong point before important conversations, so your best thinking arrives on time.
When you don’t know something, say what you’ll find out and by when. That reads as competence, not a gap.
Stop apologizing for questions. Ask them plainly.
Find one person who has seen your work and will speak for it. Their word changes how others listen to you.