Mostly because of utterly insane injections of cash into the economy, whether by unsafe lending practices creating money out of thin air (which ended up causing 2008ff) or by the central banks creating money out of thin air and buying up assets.
Now, especially after two years of pandemic-related bailouts, there's no government whose balance sheet hasn't been hit hard... governments can't take up more debt to inject into the economy, and the central banks cannot because inflation is already skyrocketing across the world.
Additionally, the people are fed up with social inequality worldwide - they saw that a lot of wealth was redistributed from the bottom to the top stock owners over the last two years. It may very well be that governments shift focus from propping up the stock markets to preventing social unrest, not to mention climate change which is bound to send a lot of companies' worth towards the bottom as their business model becomes outdated (e.g. car manufacturers, oil/gas companies).
The startups who are paranoid of the markets and prepare in advance for profitability will continue to survive and the startups constantly raising money each week and making little to no revenue will be caught swimming naked when the market retraces. Fast is a prime example of this.
We could have another dotcom style crash and it may take another 3 - 4 years to get back to the same all time highs again where that is still possible.
I wouldn’t be surprised if we just go sideways for a year or two in the market as fed tries to balance inflation and growth to get out of the Covid bubble without a disaster