Let's not get into real world science here. Other races can be smarter because of physical reasons that'd we'd understand, physical reasons that are beyond real-world humanity's current understanding, or simply "Because Magic." E.g., elves, dragons, etc. are inherently magical.
I'm getting my knowledge from forgottenrealms.fandom.com/wiki/Mystra -> "a peasant girl with rudimentary magical training but an aptitude for spells became the new Goddess of Magic"
(I am assuming this peasant girl is human).
Humans have a higher concentration of powerfull wizards than probably every other race combined. You can try to bring up population size being the reason but by that logic we should see dozens of goblin archwizards.
Humans also have the most high end sorcerers, by far.
The greatest feats in magic have been accomplished by humans.
Mystra is the reason anyone can use the wave at all. But yes she does favor humans, she's had numberous human arch mage Paramores
It is still valid to consider population size combined with WotC's propensity to make humans famous. The lack of powerful goblin wizards is even more easily explained: they're seen as vermin and thus are often killed, their societies are more chaotic which inhibits wizardly growth, and they are a historically (which matters, given we're talking about characters in lore) Evil race. Finally, are goblins more populous than humans?
Humans have the most # of high end sorcerers for arguably the same reasons: they're the most populous species, they are written about more often, and their society is so organized that people are able to go to wizard schools and gain power. Same for "greatest feats of magic have been accomplished by humans."
I haven't seen it stated in the D&D lore that Mystra is specifically favoring humans. If you have a source, I'd be interested in reading it.
But if it is true that Mystra favors humans, it doesn't necessarily follow that humans have equal intelligence compared to other races. She could just be giving them more tools, spell knowledge, etc to more easily succeed. And once you become sufficiently advanced as a wizard, you can easily use your power to gain even more power. Unless you (or the official 5e lore) is specifically stating that "Mystra raises the intelligence of humans"..?
