My sad theory

Mother Earth is trying to diminish the number of human beings in our planet before our collective greed and stupidity ruin everything.

Some of her attempts have been very peaceful: subtly causing social changes that could reasonably have been expected to drastically reduce birthrates.
There has been some reduction, but enough people have stubbornly insisted on having kids to foil those gentle plans.

Other attempts have been violent: wars, epidemics... but the human population keeps growing, at everyone else's expense.

You don't have to look far to see various ways that Mother Earth is culling the human population today.

Told you, it's a sad theory...