Because everything is crazy expensive currently. We‘re in a bubble. Look at the valuation of WD-40, Bitcoin or Shopify for example. WD-40 had a 500k annual revenue and is valued at 2.6 billion. Their free cash flow is only about 100k, so they would need 26 years to buy all stocks back at the current valuation assuming no growth in earnings.

As another example look at the current PE ratio of the companies in the S&P 500: https://www.macrotrends.net/2577/sp-500-pe-ratio-price-to-ea.... This shows that prices are unusually high compared to earnings.