Because that's how it works.
But even someone of average intelligence will be hard pressed to make a decent living the way things are going.
Apparently a better living than anywhere else on the planet doesn't qualify as a "decent living"? Aren't you setting the bar a little high? Did no one in the 19th century make a decent living since none of them had a television?
Most "average" people in the US do very well by any reasonable standards, and that hasn't been changing. What has changed is that people want to live well without any of the effort and creativity that got us here in the first place. Average people with discipline and good sense can retire well before they collect Social Security in this country. The only thing you can argue has changed is that we are losing our discipline and good sense.