It's because the story wasn't about him
The story was not in his perspective, but that does not mean the story wasn't about him. In fact, the story revolved around him and the ml. Also, just to clarify, the story was incomplete as there was not any explanation for anything he did and all he was in this story was a plot device without much character or development, even though he was a pivotal part in what made this story run.
Read again, the story isn't about two characters, it's about Seungwoon remembering and letting go. Don't focus on Haein, he was just someone from the past who had an impact but is no longer there, he is not a character that develops, he's a memory.
Everything is from mc pov, Haein there was just the idea he had of him, we were reading Seungwoon's story of them. And the feeling that we have of helplessness, of not getting clousure is the same he experienced when Haein left. This is complete.
Oh sorry I meant felt* incomplete not was incomplete (as I mentioned in my original post) and I just personally felt that this story was incomplete because as a reader I didn't get any closure* and that's probably what the author was going for, but I am just stating my opinion that it felt too incomplete for me which made me unhappy with the ending. Additionally, I came into this story hoping they would end up happy together which is specifically why I wanted more development from Haein and why I was disappointed (again, just a personal opinion)
I know the author was going for an open ending but the story just feels incomplete because we learn much of anything of Haein. We didn't learn the reasons for his actions or his true feelings so that was pretty disappointing (●￣(ｴ)￣●)