On the body of your question you say
But what I'd like to know is why would it be "impossible" to make good characters using real people.
I believe it is possible to use real people as characters, and that it is even desirable to make characters feel like they're real people, so I cannot answer that question.
However, you then close your question with
Can real people really be good characters?
Yes, they can. My genre of choice is historical fiction, and while many writers create characters from bygone eras, many choose historical people and use them as their characters, main ones or not.
I think the topic has two factors that lead to that (IMO) false statement that real people cannot make good characters.
First, as your research brought up, many people believe that
"real people are boring characters".
You saw this as a reflection of using boring people to create characters. Perhaps, but I disagree. Real people are complex, and to read a character, whether it's based on a real person or not, that can convey that complexity is both exciting and inspiring. Cardboard and one-dimension, maybe even two-dimension, characters are boring (at least IMO). But it takes a good writer to write a complex, true-to-life character.
But perhaps you know this young man who is the epithome of boring. He watches TV thoghtlessly and plays video games as if every move were automatic reflexes. He has no opinion on anything and looks moronic whenever he's asked anything. Boring indeed. To the onlooker, at least. If one could listen in on his thoughts, if one could discover his inner struggles and fears, the dramas he keeps hidden from the world, perhaps his boredom would become compelling. A good writer can make the most boring person/character feel interesting. One just has to find the right angle.
Again, (and in response to @cloudchaser who touches upon a very interesting point), characters that are true-to-life in their complexity, whether it's a literary or a genre tale, are the most interesting to me. The more simplified a character is, the more boring and annoying it becomes. Again, as @cloudchaser points out, many beginners are too close to their characters and fail to create compelling characters.
Where does the myth come from, then? @cloudchaser touches the wound, in my opinion. Some writers look at real people they know and incorporate them in their writing... but do so badly. They create such characters based on one, maybe two dimensions, of the person they know. In reality, they are creating a cardboard character and overlooking what makes that person a real person. A person is more than the manneirisms or the behaviours one witnesses. If that is the only thing a writer can copy from the muse to the creation, then the creation is going to be lacking and boring.
However, when people say that real people are boring, I think they're mostly saying that their lives are boring, and I suppose it could be true. I suppose some real life people have boring lives from the day they are born to the day they die. At least they sound boring to an onlooker, but I'm pretty sure that if a stranger looked at our lives, they would find them boring. We ourselves may find our daily grind boring and confuse that for a boring life. But think about the hardships you yourself have faced. Perhaps most days of your lives are boring, but there are also plenty of moments of tension, of drama, of tragedy, of comedy, of victory... A life can be boring, but there are plenty of exciting episodes even in the most boring life.
And then, of course, there are people who have exciting lives of exciting episodes on top of interesting episodes. Say, a Churchill. A Leonardo da Vinci (although all those hours spent painting sound extremely boring to me). An anonymous mercenary in the Italian Renascence.
That real people make poor characters is a myth, IMO. Assuming that the myth really focuses on the people/characters rather than their lives (which is another story altogether), how can one state that real people are boring to read about? Surely most readers don't think that their fears, their love and hate, their ambitions, their very selves as real people that they are, most readers don't deem themselves boring, do they? I mean, my life may be boring as a whole (despite very intense episodes that might make compelling reads), but I don't see myself as boring. The intensity of my emotions, thoughts, desires, longings... the very importance of 'me' against the world makes 'me' a compelling character to read about. But it takes a good writer to be able to transport a real person into a tale and maintain them interesting.