My theory is that (over)population might have something to do with it. The population of the US was around 150 million, it's more than double that now. Maybe th

My theory is that (over)population might have something to do with it. The population of the US was around 150 million, it's more than double that now. Maybe there was the same amount of "opportunity" back then as now, but now more people compete over it (and over the same resources). The total population of the world was 2.5 billion back then, now it's almost triple that. The world hasn't grown 3 times since then, it's the same size.