When the gods created the mortal realms, they constructed them within self-contained material spheres. These spheres protect that worlds reality from the void, the chaotic realm that exists outside of it. Within these spheres, the race of humanity can flourish. These boundaries must exist because reality is anathema to chaos. Forces within the void seek to enter the mortal realms and return them to their original chaotic soup.
These realms have self-repair units that repair various cracks that appear within this sphere, preventing the outside from leaking in. This repair system can be similar to anti-virus software (Norton, windows defender, malwarebytes, etc), where it cleans up any residue or some element that doesn't belong that can corrupt reality. Occasionally, through human foolishness or void shenanigans, a crack becomes large enough for the void's energies to enter a realm and infect the area with its energies. These create the "badlands" of the world, where the laws of reality break down and various evil creatures develop.
Reality will ultimately repair this crack, preventing further energies from leaking through and weakening its hold in reality. It also cleans up whatever residue remaining, further weakening the effects. However, the taint can never completely disappear, and the badlands that have already been infected will continue to breed evil creatures as well as defy laws of reality, such as gravity, energy conservation, etc. Why would this be the case?