There is just no where else to park money in the amounts the markets currently service. That is it, simply that there is so much capital just sitting around in the market that it becomes a self fulfilling cycle.

Market prices are based off of amount of money put in the market --> There is not another place to get returns on investment as good as the market --> More money is put into the market (stock buy backs instead of hiring for example) --> Markets rise to "new all time highs" --> Those with assets in the market gain as prices are based off the amount of money put in the market --> Those gains have to be used in some investment (otherwise they erode due to inflation) --> There is not another place to get returns on investment as good as the market.... And loop over and over.

The markets are now de laminated from the real economic reality, and as they grow they become more so. Investments in real things (and not wild schemes like crypto and LLMs) becomes less and less desirable compared to just putting the money back into the market. Bonds and other securities used to compete with the stock market and worked as a grounding force to the market, but not anymore. So now all the capital is off in la la land and since the market can't crash if there is more money always being put into the market, the market will see "all time highs" as long as there is money to be put into the market. How does this cycle end? No idea really, you could have something else show up that gives better returns making the markets finally crash or you could have this continue until functionally money will not exist outside of the market (both situations are so wildly outlandish that it seems nothing will change).