It’s important to remember that the Warhammer Fantasy world is not Terra. Presumably, no alien race has ever set foot there except by invitation. So the Warhammer Fantasy game is not actually taking place in early human history, rather the opposite.
The answer to your question has to do with the evolution of the Warhammer game. At the moment, Games Workshop claims that there is no connection between the Warhammer Fantasy and Warhammer 40,000 universes, and that any similarities between the two are mere coincidences. It is true that the games have become more distinct and less alike with the recent editions. However, it was not always so. In the previous editions, during the Rogue Trader era I believe, the Warhammer Fantasy world was actually said to be located in the 40,000 universe, but remained cut off from the rest of the galaxy by huge and powerful warp storms that made interstellar communication and travel impossible. For some reason the birth of Slaanesh, which ended the Age of Strife on Terra, did not free this planet, and its inhabitants of humans, Eldar and Orks slowly lost their technological knowledge over the following ten thousand years, retaining only their faith and certain legends about the outside world. The rules reflected this by actually allowing for some 40,000 weapons and characters to be used in Warhammer Fantasy games as ancient relics or unfortunate visitors who got trapped in the warp storm and couldn’t leave the planet again.
Before this version, though, in the very old days, I’ve been told that the Warhammer Fantasy world was actually supposed to represent the last battleground between all the races, happening many thousand years
after Warhammer 40,000, when the galaxy had been wiped clean of all life by the cataclysmic weapons of the Final War, except for this one planet where the post-apocalyptic descendents of the star-spanning empires fought over the last inhabitable world in the galaxy. Presumably their use of primitive weaponry is a reflection of the famous Einstein quote:
“I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.�
Finally, it could also be that the Warhammer Fantasy world is just another world in the 40,000 universe, not cut off but undiscovered. Remember that certain Eldar factions, such as the Exodites, felt that the only way to truly protect themselves from Slaanesh and prevent another Fall was to go feral and live off the wild, so to speak. Most Exodites in the fluff have retained some technology, although not much, but it’s not impossible that the Elves in the Warhammer Fantasy world are actually Eldar Exodites who do possess the technological knowledge to create greater weapons than bows and arrows, but refuse to do so because they fear the consequences of possessing such great power again. As for the human’s level of technology, it’s been proven numerous times that they will quickly lose all technological knowledge if cut off from the rest of their race, as seen during the Age of Strife, and the Orks actually have the special feature that their level of technology reflects their opponents'. If he uses a bow, they do too. If he uses star ships and giant laser weapons, then they do too. It has got something to do with their creation, I believe, although that is an entirely different discussion.
Hope that helped to clear up the confusion!
~Grephaun.