Because magic exist in D&D words. Without magic, your average human is never going to get higher than 5th level. Period. Albert Einstein was 4th. King Arthur was maybe 5th, and he had magical help. Anyone who reaches level 4 is already likely well above average for a human. Anyone who reaches level 6 is pretty much superhuman.
So, you are basically assuming that the world of D&D somehow makes people exceptional. This is a fairly bold claim, and one that is probably untrue. Not only does the D&D world probably hold much smaller population than the current world, it is also much, much more probable for a person there to die young. If level 5 was a paragon of success, the strongest man alive, how many dragons would want to kill that man out of malice or spite? How many gods would turn on him? How many illithids would want a tasty snack?
How many creatures much, much more powerful than him would kill him, just because he's a paragon of success.
For characters in D&D to realistically ever reach levels above 5, we have to assume there are multiple of them. And I really mean a lot. We will have to assume a ratio of heroes to zeroes that is mathematically incomprehensible. Only explanation for this that you have to give is "it's magic", which does not really hold. Even with magical help, there won't be a way to plunge people into superhumanity.
No, because in the modern world we cooperate and use technology and industry to do what fantasy worlds do with magic and personal prowess. The gun superseded the bow, not because it was better (it took several centuries for a gun to even match a bow in most respects) but because it was easy to learn. Bows took a lifetime to master, guns took two weeks to learn to fire competently. IRL is gun using. D&D is bow using. In D&D lifetime study is where the power is at, because there are no mass producible implements of power.
But even in real life, people study for lifetimes. Physicists devote their lifes to research, and artists paint night and day. Athletes are trained, often through brutal and life-risking methods, to compete. Michelangelo painted for his life, to the point where he neglegted his personal hygiene to get more time to paint.
Face it, if we are to use D&D as a statistic starting point, one in 216 people would have 18 in any given stat. 2.745825342% of people would have 18 in at least one stat. That's hundreds, if not thousands, in your hometown already. It is very probable at least one of them in your hometown alone has the dedication to master any art or craft or research. These people are not incredibly rare. How many people your public school had? 300? We're looking at 8 people like this. 8.237476026 to be precise.
Not only this, but back when we didn't have the technology, people were living in what was more or less the D&D world, only without great wyrm dragons capable of destroying the entire human civilization just for the lulz. Without trolls wiping out villages alone. In a lot safer world where they had more time to practice their craft. In a world, where, by all regards, they should've gotten better at their respective outlet of energy than their D&D respective would.
If anything, the D&D world should produce less paragons, not more. You seem to claim the contrary is true, and to justify this with the presence of magic, when it is fairly obvious to not be true. Not only this, but real life has demonstrated that best works of art are not necessarily a result of long practice, but of short flashes of insight. Mozart died how old again? Newton just happened to sit below a tree. That's a natural 20 right there, and not surprisingly, natural 20's happen 5% of the time, and you'll be rolling a lot of those checks. A lot. Now, let's model Newton as.. Level 3 expert, with three ranks in mathemathics, just to be fair. I mean, he wasn't trained a physicist, definitely not to a degree the modern people are. So, assume int of 18, and we are looking at 27 result.
Our only way to model superhuman successes is through "complex" skill checks, where we are rolling hundreds of d20's, to reach a point where it is statistically reasonable that no one came up with these theories prior to these people. Lifetime devotion leads to success because you roll enough to increase your probability slightly. For every succesfull person with lifetime devotion, there are hundreds more that accomplish nothing, because of the complexity of these skill checks. They might have to roll a natural 20 twenty times in row to get a masterpiece, and if they only craft a few hundred paintings, chances are none of them are that great.
Still, the same is true of D&D. Most people should be level 1, with almost zero outliers. Skill check of +11(4Ranks+4Int+SF) is enough for great success if you have a long-enough lucky streak and maybe get a circumstance bonus. Chances are Einstein was just a level 1 physicist. Still, humans in current world should be producing more "miracles" than the men of the past.
I cannot fathom a reasoning that would lead to D&D world having an abnormal amount of heroes. It's way too improbable from a statistical viewpoint, and explaining things based on "it's possible" doesn't make sense. You have to compare the two worlds, and try to find a reason why D&D would be special compared to our world. After my demonstration of the contrary, burden of proof lies on you, and no, "it's magic" doesn't count.