As much as this argument is actually rather a good question, games dont take into account curvature for a round planet generally because its just a detail, why have it curved as it does actually take more work then needed to add a pointless detail, and adding more coding into a program could just cause more bugs later on when they update or change said game causing problems with the said coding.
World of Warcraft for example is a VERY large mapped game with multiple zones that makes the planet wide and large, including three continents so far on the world of -azeroth- as its called, they dont add curvature in the distance because the zones eventually end out at sea or the ocean. However its widely known the planet in World of Warcraft is spherical from multiple models of the planet actually inside the game including in the dungeons and in other areas where actual globes of the planet are found. The reason they dont add curvature is because the length of the planet must eventually end in the game and cant continuously sphere around itself.
That would be a simulation of the fabric of space-time bending back upon itself.
I lol'd, I just had to say that.