There has to be enough spending money in the system

It comes down to something very simple: Supply and Demand.

Demand = want + ability to pay
Demand is the driver. If there isn't enough money looking to be spent, that is, if there aren't enough people who want things and have money to spend, you won't have enough demand. It doesn't matter how much money is in the hands of the wealthy to invest and create businesses. Usually people think of demand as "wants" but you also have to have the ability to pay for these wants to actually create economic demand.

Bubble = investment inflation = too much money chasing too few investments
If not enough people can afford the products or services, the wealthy won't invest and the money goes into non-productive investments. Too much money chasing too few investment opportunities will create inflation of the price of the investments, AKA a bubble. Sound familiar? So, if too much of the money is in too few hands, there just isn't enough money in the system to make it work.

People gotta be able to afford your products
Henry Ford saw this back in the 20s. He saw that with mass production, he would be making a lot of cars. In order be successful, he needed lots of people who could buy his cars. He tripled his factory worker's salaries--basically so they could afford his cars. So it's not a new insight, it's just tough to figure out how to achieve the right balance.

Money makes the world go 'round--but not if only a few have most of it
The middle class needs to make enough money to drive the system. We have gone out of balance. This lack of spending money in the system has been a problem for a while, but we stumbled along by making credit easy. We weren't paying people enough, so we made play money readily available. We all know how well that worked out.

Our economy is out of whack
After WWII, the great expansion was driven by large demand because most people had money to spend. The rich were nowhere near as rich as they are now. Before I did some research, I found this hard to believe, myself, but I found that because of high taxes and the strength of labor unions, the top 1% only had about 10% of the income. From the 70s on, the rich kept getting richer, and now the top 1% have over 25% of the income. Somethings gone way out of whack.