People thinking others are "just stupid" indicates that they are avoiding thinking by jumping to conclusions.
There are no smart or stupid people, but people being smart or stupid. People who say "they are just so stupid" usually mean that they aren't being smart.
Being smart means thinking things through. Use first-principle thinking to find the real answer and second-layer thinking to take people's motivation and background into consideration.
Being stupid means avoiding thinking by jumping to conclusions. Searching for the real answer is too painful, and labeling people as "just stupid" is an acceptable explanation.
"Saying I don't know" is underrated because it takes intellectual honesty to draw the line between the known and unknown.
This reminds me of Wittgenstein's ruler (#31.1): If you don't have confidence in the ruler's reliability, when using the ruler to measure a table, you may also be using the table to measure the ruler.