It's the derivative cycle. It's not just Manhwa, it's Manga, anime, comics, movie, etc. When you first start out everything is new and interesting. As you dive deeper you see the patterns and start to get jaded. Once you get bored, you'll have to wait ten years for the derivatives cultural drift to change enough to "feel" fresh.
Because unlike what you think, creativity isn't a vacuum. Its a long chain of inspiration, derivatives, and parody of previous works. Orcs, goblins, and Elves have been around centuries before Tolkien. Who took inspiration from the likes of Beowulf and Fae myths.
PS: I myself drifted from US comics and cartoons, to anime, then manga, then Manhwa, then webnovels, both JP, KR, CN translated and eng OGs. Gaming wise, I went from platformers, to rpgs, to fps, to rpgs again, to mmos, to metroidvania, to crafting/survival, to zombie. It takes time to see something new and interesting in all media after while.