Because he's fighting the other creators armies.
So the creator that made the rulers and monarchs had fun seeing his creations fight endlessly until the shadow monarch killed him. Later on, Sung Jinwoo killed all the monarchs, and regressed time to allow people have a normal life while he fought the monarchs alone and he killed them all (that's why we see that now his son is going to meet their descendants, because the monarchs lost the fight).
But now the superior beings, the creators, are trying to mess with Earth while sending their own armies, their own creations.
So if it took a long time for Jinwoo to win the fight against the monarchs, imagine having to fight against a hundred similar beings that command armies of hundred of thousands.
Maybe at the end of eliminating all the creators creations and having them become shadows, Jinwoo will finish the creators themselves ╮( ̄▽ ̄)╭
Was not Sung suppose to be the strongest, so how come he take so long to kill all his enemy. And why does Cha Hae-In has to be missing instead of being with his son, unless she got kidnapped. Not sure if she kidnapped by the enemy since it seem like ant also doesn't know that Cha was missing until her son ask about it. Also, i wonder what happened to the people that Sung used to know are doing. I'm just curious. The story is good, and I like his appear when he was possess by Fang. Which I like it if he have power that is a little different then his dad, it would be interesting.