The gates etc. has already happened multiple times, but each time lead to too much destruction, but the semi-good demi-gods have an item that resets time (which is starting to lose power, so it can't be used indefinetly), and demigods are the only ones that remember everything that happened before the time rewind.
After Jin-Woo wins, he asks them to reset the time once again, and wipes the ground with the evil demigods in a sub-space they were hiding (over long years of multiple guerilla battles which he wins, raises the dead opponents, and flees. He does it for so long that in the end he has bigger army that the evil-demigods). As Earth is not the battleground after this reset, there was no need for gates to be created to strengthen the planet to survive the battle between the demigods, so there are no hunters, and since time was rewinded, nobody remembers what happened.