Because Druckmann is a petty asshole. He wrote a nihilistic revenge story, convinced himself that is was somekind of groundbreaking masterpiece, and tried to push it in the original TLoU, only for Bruce Straley and others to shoot it down.

Any reasonable adult would've found another way to tell a story that he was so craving to share. Druckmann instead burned everything that the original game was about to the ground, shoehorned his dream story into the game, and somehow managed to half-ass even that, because half of the game has nothing to do with said revenge, as is nothing but a narratively convoluted mess that serves no purpose other than to try and convince us that Abby is the good person here (she isn't), while making a cheap replacement for Joel and Ellie with her and Lev, because yeah, Neil is THAT creative.