
because the writer wanted to give him an arc about heartbreak!! after all, he is one of the 3 main characters.
this comment is a really great analysis of him: http://www.mangago.zone/home/mangatopic/9859606/
honestly love or hate isn't your usually romance where the one and only point is for the main 2 ppl to end up together and live happily ever after. it's clear the author wanted to explore different themes and facets to love and relationships.

fr tho. taku was never bound to experience full happiness with haesoo cause haesoo was always thinking about someone else. atleast now he was able to travel, be more immersed in photography, and even found a new lover who doesnt make him question whether they'll still be together the next day or not. thats better than nothing. yall just being petty and it isnt attractive
So u guys seriously used taku character to build the plot and throw him away, just like that,how can you do that dear author :')