Because he thought having a protagonist tell the whole story, through dialogue, that should’ve just been left as narration was a good idea. And he thought it was another good idea for the protagonist-narrator speak just like Joseph Conrad writes, which is nearly impossible given for anyone, much less someone not trained in elocution, large-content memorization, and pathologically inclined to break basic social etiquette. And because he thought other in-story characters wouldn’t just get up and drown themselves in the disease-riddled Congo after he told maybe a few pages’ worth of his past and write him off as crazy and not just an oddball. And because he thought readers wouldn’t be wondering when the guy is going to shut the hell up and get back to the present and get on with the story. At least Ayn Rand had the good sense to make chapter-long speeches actual speeches and not impossibly well-constructed oral storytelling rants from a 19th/20th century seaman.