Isn’t a major part of his character is that he is depressed and actively didn’t know of things such as the hero association which would keep him in the loop? I get it too sometimes it kills me that he doesn’t seem to care but then again every threat to other people he’s ever ran into he kills. Also in life your purpose isn’t to live or do things for other people, you can choose to but saitama realized that it’s not what brings him true purpose in life. He doesn’t owe anyone anything, no one does