Because it makes characters more interesting, flawless characters are honestly boring
I can't say I agree, besides, who are we considering a "flawless" character at any rate? A lot of the time presentations of flawless heroes are boring because I simply disagree with the assertion, hence people's impression that "flawless" is boring.
Superman, for example.. pretty flawed yes? For starters he continuously misrepresent himself and basically lies to characters at pretty much every turn. In fact, he's kind of a violent sociopaths who beats the **** out of everyone he comes across to teach them various lessons. I'm not persuaded of his reasoning or humanity.
Compare to Gladiator (which I heard Casey Hudson liked), how flawed is Maximus? He kills and destroys, sure, but it's primarily either in service to the empire or in a struggle for his own life.
I think the last time BioWare wrote a flawless character was probably Ajantis in BG1, and that's probably just because the BG1 characters didn't have enough personality to make the resident KISA's flaws clear.
A fact I am slowly realizing, the Bioware I liked was possibly just an accident from their perspective. And yeah, I did like Ajantis.
Since the dawning of modern fiction. Complex characters are designed to have flaws so that they're relatable, so that they aren't boring and predictable and godlike. I could cite to you literally (I don't use the word "literally" lightly) hundreds of characters who are beloved as much because of their flaws as in spite of them. Scarlett O'Hara in Gone with the Wind, Oskar Schindler in Schindler's List, Charles Foster Kane in Citizen Kane, John McClane in Die Hard, Sophie Zawistowski in Sophie's choice (though, that's as much Meryl Streep's acting as anything).
Dorian's flaws, his multitude of humanizing flaws, are written into his character intentionally. Plenty of his fans love him while being perfectly aware of those flaws.
The quote Vertigomez referenced answers that question as eloquently as anyone could, I believe.
Maybe characters should be designed to be incredibly so they inspire fear and intimidation, as well as respect? All the characters you mentioned have at least some extremely razor-sharpened edges which makes them powerful and inspirational to people. Scarlett O'Hara doesn't take **** from anyone and ignores the societal norms to do what she wants. Oskar Schindler is brilliantly rich and calculating. Charles Foster Kane I don't know because Citizen Kane was boring. John McClane is clever and tactical. Sophie's Choice also didn't see.
Edit: Plus as someone else demonstrated below, perhaps whatever "weaknesses" they have are what make them unappealing to people on some level.