If you look at D&D there's no indication in the rules that an 18 INT makes the character a smartest person in the world master of a dozen languages discoverer of fundamental mathematics. They can memorize some extra spells. Period.
It's both a little more and a little less than that. The biggest factor in determining a D&D character's competence is Experience Level. A 10th-level Wizard with a 15 INT can memorize way more spells than a 1st-level Wizard with an 18. The only base characteristic that's ever used to measure something concrete is Strength and Carrying Capacity/Encumbrance. That is, a 1st-level character with an 18 STR is actually stronger than a 10th-level character with a 15 STR, but only in one, specific way; in every other measure of strength, the 10th-level character is stronger.
Also, most of the things being cited as measures of Intelligence are, in D&D, skills: In some versions of D&D, the number of languages you speak is a function of your race. In 5th ed. D&D, languages are a function of your character's Background and INT isn't even a variable. A modern Master's Degree in something would be a skill specialization and/or experience level - depending on the skill, INT would be a modifier. And, for all that, the roll of the 20-sided die is still be the biggest thing separating a skill contest between a person with a 15 and a person with an 18 in the relevant characteristic.
Yeah CFC and D&D forums tend to skew a bit higher intelligence wise.
Dumbasses don't tend to play strategy games.
Yes, but it's more than that: Playing strategy games makes you smarter. In the same way, it's not totally accurate to say that fit and strong people hang out in gyms. They do, but it's the spending time in the gym that helped make them fit or strong in the first place.
I always figured it was people as a whole. If all the rolls sucked that was "someone who just wouldn't go adventuring" and allowed a reroll.
That's how it's commonly done, yes. As mentioned, "4d6, use the best 3" is a common method for separating the adventurers from the normies, as are point-distribution and point-purchase methods (which take the randomness out, and let you just decide what kind of character to play).
The problem with this is the underlying assumption that the reason some people can't [climb a tree] just is because they were born that way. A better illustration would be several chimps, but one of them is given climbing gear, one of them is blindfolded, one of them has her hands tied behind her back, one of them has a leg in a cast, and they're all being asked to climb a different tree (one of them is a palm with no branches, for example).
