I don't really know, but as far as actual fertility(the ability to become pregnant, rather than just birth rates (the rate at which people have children). It is more likely to be down to pollution like microplastics and lead in the water and the like.

Possibly even rising temperatures, don't spermatozoa die above 78°F/25.5°C? So if men can't cool off their junk, they can't get anything worthwhile delivered to the egg? I'm no biologist so I'm iffy on that one.