What has changed since 1973? more junk food and fast food, more processed food than ever, less exercise, mobile phones and computers with the associated EMF and RF pollution which we can’t see but has an effect nevertheless. And then there are the micro plastics in our environment which we also are ingesting as everything is packaged in plastic. So we DO know what is causing the fall in sperm count.
On the positive side, a reduction in world population is not a bad thing and will give nature a chance to re balance itself.