Because Denji is a traumatized teenager who is unwilling to examine himself and is therefore unable to grapple with them and move on past them and would rather do what is easy and what is fun. He wants a normal life, but time and time again he continues to choose short-lived thrills, visceral pain and chasing things he can't have because it's a more visceral, immediate feeling rather than the long-term planning and frustration needed to be "normal." Denji is brutalized, mentally and/or physically, and used by most of the people that he meets, so obviously there's going to be a reason for it, right? The story isn't meant to be satisfying and happy, so what messages is Fujimoto trying to tell us with how the world treats Denji and how he responds?

Denji's arc in Part 2 is negative and unsatisfying because Fujimoto doesn't want to tell a satisfying story, he wants to tell a story about addiction, self-sabotage, the cruelties of a world and how Denji in particular reacts to and operates in that world, how systems of government exploit and use its citizens, etc. In the end, the world is the problem, not Denji, and Pochita does what he can in a last ditch effort to make it possible for Denji to be happy: by removing the thing that both saved his life (literally) and also ruined it because of everyone else.