Well, having dead parents in manhwa can heighten the emotional impact. It gives the characters a tragic backstory and makes the readers sympathize with them. Also, it simplifies the character's family dynamics and focuses more on their personal journey.
Maybe it's to create a sense of hardship and make the characters more resilient. It also gives them a motivation to strive for something better. Sometimes, it's just a way to make the story more engaging and unique.
It's a common trope to add drama and character development. Without parents, characters often have to face challenges alone and grow stronger.