we evolved from apes. apes, while primates, are fundamentally different from monkeys.
the way evolution works is that certain animals gain certain features over time to better suit their environment. keyword there is "to better suit their environments"
monkeys work just fine for jungles, but in the african savannah (where humans did most of their evolution) required the apes that lived there to develop the muscles necessary to stand upright so that they could run longer, which then evolved into the prehensile thumb for better dexterity, which eventually became humankind as we know it today. the only reason that humans are the most widespread of the great apes is that we got lucky with sentience