Lots of ways you could handle that. You could say that humans have a lot more people who are adventurer material, and outnumber the long-lived races. Or that the long-lived races have a smaller percentage of those who can AND are willing to risk their lives to do the level-grinding. Or that human societies are (however unevenly you make it work) more accepting of social climbing by means of acquiring gold, fame, and power, and so there's more point to it when a 14-level human fighter can pal around the king, and a 14-level elf fighter is jeered at as pushy.
Or, of course, you could say that it's true. Humans are marginal beings, pushed to the edges of civilization by the greater powers of the long-lived races -- except of course, where are the monsters?
Humans would be the ones fighting against their raids. They would also be the ones organizing counter-attacks and attempting to wipe out dangers. Members of other races would have to go to human lands to adventure and level-grind, and therefore at least have to tolerate humans -- all the more in that humans probably have all the merchants who stock adventuring gear. And soon also be the best source of adventuring specific magic. Liking for humans, long familiarity, simple length of time, and even the social climbing advantages that I mentioned above might have non-human adventurers retiring to human lands, teaching humans their tricks, and fighting to aid them in times of peril.
Meanwhile, the humans would be accumulating the bulk of the magical objects, and a disproportionate number among those of the ones that increase ability at combat, both as loot and as the ones who need such objects.
Give a few centuries. Maybe a millennia. Perhaps the next thing your five-hundred-year-old elf king learns is that the humans expelled from elven lands in his childhood are now ready to crush his lands and reduce them all to vassals.