Note that "stock prices" don't exist as a generalized statement. Individual stocks exist, and some of those may be at a record high, but most stocks aren't. Are you thinking of "these are the highest value companies we could aggregate" index funds, like the S&P 500 or the like?
As for the answer: because any unregulated commodity market will see that commodity priced based on demand, and demand creates demand.
It's not that hard. We printed dollars in record numbers (bipartisan - both Trump and Biden).
Of course assets will pop in dollar-terms. Fed's interest rate controls are bound to ease up, as the US economy is _addicted_ to getting injected with printed fiat currency.
As for the answer: because any unregulated commodity market will see that commodity priced based on demand, and demand creates demand.