TwoSix
Everyone's literal second-favorite poster
Because those differences can be true in aggregate but not true in an individual. Like, for example, it's scientifically true to say that people of Asian descent have a much higher chance of having the B blood type than people of European descent, just like it's true to say that Europeans have a much higher chance of having the A blood type. But, the majority of people of those descents still don't have those blood types.i don't see a need to define their mental abilities as totally alien or abberant to still be baseline different from human's to a noticable degree, there are many observable differences in human individuals capacities to process different types of information and it's not like those people are biologically incompatable so why wouldn't the same be true across humans, elves and orcs? even if one is between baselines and the other is between variance around the baseline.
Now, let's say in a fantasy world we instead have some gene that gives high agility (a baseline 16 Dex) or high intellect (a baseline 16 Int). It could be totally true that a third of Elves have that high agility gene, and thus have a much higher baseline of Dex (a 16 vs a 10) than other races or cultures. You could even average that out to say that Elves, in aggregate, have a +2 Dex compared to other races. But that "+2 Dex" doesn't have any bearing to any one individual elf.
Most genetic traits are going to have that kind of distribution. It would be extremely rare to have a genetic trait that's extremely common in one population but not expressed at all in other populations (which is what the +2 stat to everyone in the population would be modeling.) But the capacities that are measured by the 6 stats are already hugely variable (thus the 3-18 range), and thus capture the kind of model we would expect to see from traits that can be influenced by a broad swath of genetic factors.
The damning part of the "+x to stat for a race" model is that even looking at the rule as a simulationist, it's a very bad simulation.