Never in a million years would I have guessed that there have only been 9000 IPOs in the last nearly half-century. Really drives home how many US businesses are privately owned.
It is surprising that it's such a small number, but upon reflection maybe not so surprising. Stock markets were invented to allow massively capital intensive businesses like railways to get off the ground. You can't grow a railway organically by reinvesting profit like a regular business; it needs to be fully built before it can bring in a penny. Naturally there can only be so many of these businesses. In the case of railways they usually become natural monopolies. So being publicly owned was a really great thing.
But most businesses don't need such large capital injections anyway. They can grow organically, and there's very little reason to sell a profitable company. Although it does happen, of course, Google being a prime example, having gone public when already profitable.
It looks like they were only looking at the NYSE and Nasdaq. Smaller companies would not qualify to trade on those exchanges, they would trade over-the-counter. There are more OTC stocks than there are that trade on NYSE/Nasdaq.
"The sample is composed of the IPOs of U.S.-based companies with an offer price of at least $5.00 and listed on the NYSE (excluding NYSE American and NYSE MKT issues after the merger in 2008) or Nasdaq (excluding Nasdaq small cap issues before October 2005 and, after Sept. 2005, Nasdaq capital market issues), excluding ADRs, unit offers, SPACs, closed-end funds, REITs, partnerships, banks and S&Ls, and stocks not listed on CRSP (CRSP includes Amex, NYSE, and NASDAQ stocks)"
