I have two personal pet theories (is that really a common idiom in english ? I like it !) on that topic.
1. since we now focus on low inflation, people have little incentive to spend/invest their money, they can just hoard cash for a long time with no risk of losing money.
2. The low imposition rate since Reagan accelerate the concentration of capital, and rich people buy assets with their money, not consuption goods.
And yes, it is somewhat common! I think your second point is definitely part of the problem. As to your first, I think that looking at inflation in these terms requires looking at interest rates as well. When both inflation and interest rates are low, hoarding cash is highly incentivized. If inflation is low and interest is high, there is a large opportunity cost to holding cash rather than collecting interest via a mechanism directly tied to the cash (e.g. a bond). If both inflation and interest are high, it would probably be optimal to invest in assets that won't have their underlying value erode as they do with bonds.
There is no indirect means to receive money, except for profit, and that it is not related to stock.
Profit is ultimately the primary means for company cash liquidity, held as retained earnings, after the company has survived its initial years of inception and growth.
Dividends enable the company to go public. The secondary market makes the stock liquid, which is convenient but not mandatory.
The emphasis on assets for retirement locks a lot of money up in an unproductive musical-chairs of asset transfership. For example let's say I have a pay-day and want to spend $100 on buying stock. When I buy that stock I'm purchasing it directly from someone else who now has $100 instead of the stock. They can spend that money on goods and services, but in my opinion there is a far greater chance that they will reinvest that money in some other asset. If the actual assets in the market don't change, I think you can say that my investment increased the total sum of costs of all assets. I'm not 100% sure this is solid logic but it's some rough reasoning.
The problem is that at any given time more people are buying new assets instead of selling them off. This is because you can make money on purchased assets through rentiership (dividends, renting, or collateral for a leveraged investment that pays dividends/can be rented), and also because people are cautious and save more than they need, since asset prices are unstable.