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.