Compared to dwarfs and elfs, we are breeding faster and in greater numbers. In time that takes elf couple to raise their kid to adulthood (say 100 years), human one already has grandkids. It's hard to kill of humans below replacement speed. Quantity has quality of it's own.
Unlike Orcs, humans are more open to cooperation and trade, at least if they see their own benefit ( we are greedy bunch). Also, while we are tribalistic, we are fast to unite against common enemy And just like stereotypical orcs and goblins, humans are capable of doing some horrific stuff. Don't underestimate hairless murder apes.
Curiosity is also one of strong suits. It's that curiosity that inspires us to innovate and progress technologically.