Technology

It's because they're concentrating all the wealth. The wealth in the US used to be far more distributed, with the majority existing in the large middle class. Reagan started the policy by Republicans to pass laws and regulations designed to benefit the wealthy at the expense of everyone else, and then Clinton got the Democrats on board with the same strategy. We're approaching the end game now where the middle and lower classes are nearly bled dry and the rich will start cannibalizing each other to be the last fattest rat in the garbage pile while the entire US economy collapses around them. Be on the lookout for the smarter rats to start fleeing the ship by transferring as much wealth as they can into foreign assets that will survive the collapse of America.

In theory the line should go up, population increases, technology allows us to become more efficient, inflation etc. That said what’s going down is at least the standard of living, employment, adjusted wages. Personally I’d put money on the AI bubble popping and crashing the market eventually but who knows how long that’ll take