Perhaps he feels betrayed by the wizarding world. Maybe some of his friends turned on him or the institutions he trusted failed him. For example, if the Ministry of Magic wrongly accused him again, it could push him to abandon it all. He may also be haunted by the many deaths he witnessed and feels that leaving is the only way to escape the painful memories.
It could be for love. He might meet someone from the non - wizarding world and decide to leave magic behind to be with that person. He would then choose a simple Muggle life, away from the complex and often dangerous wizarding society.
In certain fanfiction scenarios, Harry could leave the wizarding world to explore other magical realms or non - magical places. He may feel that his growth and understanding of magic and life are restricted within the wizarding world he knows. Maybe he meets a group of people from outside the wizarding world who offer him a new perspective and a chance at a different kind of life.
Well, it might also lead to a split in the wizarding community. Some might side with Ron and Hermione, believing they had a good reason to abandon Harry. Others would be outraged and try to find Harry and bring him back. This could create chaos and internal conflict within the wizarding world, making it vulnerable to outside threats as well.
Perhaps he leaves in search of a more peaceful life. The wizarding world, despite its magic, has been filled with turmoil for him. He could be seeking a place where he can simply be Harry, not 'The Boy Who Lived' or the hero who defeated Voldemort. He may also be on a quest to discover more about his family's past in the Muggle world, something that has been somewhat overlooked in his life in the wizarding world.
One reason could be that he's tired of the constant danger and battles. After all he's been through, he might just want a normal, peaceful life away from magic. Maybe he wants to start a family in the Muggle world where his kids can grow up without the threat of dark wizards.
In some cases, he could be leaving in search of a greater purpose. He might feel that there are things he can learn or achieve outside the wizarding world that will ultimately help the wizarding community more. He may also leave because of some prophetic vision or a feeling that he doesn't belong there anymore, that his path lies elsewhere, away from the familiar magical realm he grew up in.
One possible reason could be that she is forced to for some dark magic related plot. Maybe an evil wizard has put a spell on her that makes her think she has to leave to protect them. Another reason might be that she feels suffocated in her life with Harry and the kids, longing for her old adventurous self and thus decides to abandon them to seek freedom.
One possible reason could be the trauma he has faced. Harry has endured a great deal of pain, from the loss of his parents to the constant battles and threats in the wizarding world. He might want to escape all that and find peace away from it.
One reason could be that he values the authenticity of the wizarding world he lived in, and fanfiction often strays from that.
One possible reason could be that he's tired of the constant danger and politics in the wizarding world. After all he's been through so much. He might just want a normal, quiet life away from magic.
One reason could be that she feels disillusioned with the wizarding world's politics. After all the wars and power struggles, she might just want a simple, Muggle life away from all the chaos.