I'll take a stab as why I think this is the case. Putting food quality discrepancies aside (food in third world countries is usually organic while GMOs are common in first world countries), I think the main reason is the level of social isolation. if you are a professional making good money, you are likely spending aot of time managing and analyzing, and the social interactions you are exposed to are usually in a formal setting. Back in the day, people interacted alot compared to todays, it was so common for people to interact with their neighbors, acquaintances and strangers on a very high level. This high social interaction and culture, forced men to be in constant interaction and in vicinity of women, which in turn stimulated the subconscious production of sperm. Also, excessive stress over prolonged periods (common in the west) is known to lower fertility in men. Although there are studies that show that women fertility actually increases under periods of stress such as wars and famine.