It's because most mangakas know what they're doing and how to structure a story, a lot of them started in apprenticeships from established authors who know what they're doing and have had literature clases.
On the other hand, authors that do manhwa don't understand how scripts work, the different types of story structures or how to conclude character arcs, they prolong most of their stories with unnecessary drama and forget to tackle the main conflicts.
Yeah... There is actual real training going on.
Sometimes you can even see from someone's style whose assistant they were. Or at least worked together with someone, even if they both were assistants for a mangaka. Of course it's often just a guess, but especially similarities how they draw emotions or comedy often stuck around when those assistants finally do their own work.
Also, mangas from novels almost always have been based on literature that is a bit more than just a web novel. New mangakas seem to start with those and then do their own thing that might not be based on a novel, if they want to.
Nevermind how many oneshots they have probably been part of, not just anthologies, but also in magazines.
And with manhwas, some payments comes by chapter nowadays, not based on volumes. So sometimes you have to make more chapters. I bet that part is fault of the companies and their system to make readers pay.
And I know the novel is slow burn too, but could they have at least cut/speed run something. This makes me miss again OG mangas, they manage put so much story just in one chapter and keep it together.
Picture can say a thousand words, so please USE THOSE PICTURES!