The only way I can make it make sense is that people skim all the interesting bits bits that aren't the main couple interacting. A bunch of the fandom is very, very young (the "yaoi-seeking children", as chromemuffin terms them), and doesn't seem to be capable of making the analytical leaps required to realize that he's smart. Or the animation de-emphasized his smarts? I dunno. I have no idea where these people are actually coming from. His blind spot is Lan Wangji's affections, so lots of people go "oh he's so dense for not spotting them", which gets simplified to "oh he's so dense", and then we get fic where an OC who is for some reason called Wei Wuxian is dense?