Why are there so many dense main character?
It’s easier to write.
People tend to overthink the answer to this question. It’s not like authors are going “Hm, you know what I think readers would enjoy? A protagonist who can’t pick up on obvious interpersonal cues.”
In a story with a romantic/relational plot or sub-plot, it’s to extend drama without making him a manipulative douchebag. If the protagonist doesn’t pick up on Love Interest’s obvious attraction, he’s a doofus. If he does pick up on it and strings her along, he’s a scumbag.
I can relate to a doofus, but I don’t want to relate to a scumbag.
In a story where those things don’t play much of a role—like in Japanese light novels, where this kind of protagonist is the standard—it’s to keep relationships from progressing so they don’t eat up too much of the story.
Now he can have flirty banter with everybody without betraying anyone OR spending precious page space on developing a healthy relationship. Maintains the status quo.
And that’s it.
You could say “Well, if you’re not going to have the main character pick up on it, don’t write other characters flirting with them in the first place,” but that can end up being distracting and weird in itself.
Ah, so your main character is a chiseled Adonis regularly dethroning evil gods in defense of the common man, and not a single person is ever attracted to him? I see, I see. Is the most attractive trait in this world bland incompetence?
Because if so, I’d like one isekai ticket, please.
That is probably the best answer to my question. It at least makes a lot of sense from an authors perspective.
After all writing compelling relationships is probably pretty hard in the first place.
I think what I dislike the most about dense main characters is that they don't seem to evolve over time in many stories.
That said bland incompetence seems to be a rather attractive trait if japanese novels are to be believed.
One of my favorite one-shot manga chapters is a guy getting isekai’d to a fantasy world with three beautiful women as party members, and he keeps reacting with frustration to how easily they’re throwing themselves at him.
“We’ve known each other two days! How are you in love with me already? Have you never met any other guys?” etc.
I was disappointed it didn’t get serialized; I would have enjoyed seeing that concept fleshed out.