It's important to note that Lambda is not the world of "Earth's fiction", it's was a world with its own language and so on, who was suddenly invaded by the Demon King from
Another Dimension, summoned some Japanese as the cliche goes, and he is defeated, and they
earn their happy ending, right?
Now, this is also important to note, before the Demon King, there were no monsters in Lambda, just like in Earth and Origin have no monsters naturally (Origin and Lambda did have Undead, but that's caused by stuff related to magic and so on), there were also no dungeons, no status system, and it was basically Utopia (And the big bad main enemy is fighting stupidly hard to return the world to that utopia)
All Monster and Dungeon stuff came with the Demon King, he made monsters to kill and cause fear on humans, they are literally made to be enemies of humankind, as their creator, the Demon King, stipulated, but the Demon King doesn't do everything alone, he has those under his control to do his binding if necessary, and that's why he had an army, and there were commanders. The Demon King wouldn't command the rabble, that's for the "slightly above rabble" to do, so he made new monsters to act with more intelligence to command the mindless idiotic monsters. The Noble Orcs are the superior version of the Orcs.
Then we have races like Ghouls and Vampires, they also didn't exist before, and they were born from the Goddess Vida as Zadiris explained.
Now, why do monsters and races have similar names to myths and lore from Earth, while being so different from them in another way? One excuse you could give is that the Champions don't come from the same world that our MC comes, which is said that, although very similar, is clearly not the same, and is unlikely to be an exact copy anyway.
Another point is something that happens more and more later on, where new races either comes with new names, or are named by Van, in both cases though, the names comes more as an approximation to what Van knows. This could also be attributed to Van's influence and the fact that he is from another world. The Status System is something from Lambda, closed to Lambda, that can't influence and be influenced by external factors normally, however, what happens when an external factor is internalized? This started with the 7 champions, that would spread knowledge and myths from their world, but surprise, the 4 that were spreading Knowledge died, and the 3 that remained were against "corrupting Lambda natural path". Now we have another internalized External Factor in Van.
But the influence on the system had already happened, and when new races appeared, approximated names were given, and maybe even it heavily influenced their abilities (which may be why Light is Vampires weakness, while not against Ghouls, even though both were born from an Undead, which is relatively weak against Light).
Of course, the main reason is convenience from the Author's perspective, since by giving some specific name, the reader will already associate with specific cases, for example: That Vampires are evil, and as we saw, Valen worked for an Evil Vampire, although he wasn't evil himself.