A better explanation is that stock market growth has slowed because real GDP growth has slowed [0], which it should match [1]. Why has it slowed? Well, economists have hotly debated this so you'll have to read into the dozens of theories but not once have I heard it is related startup IPO time.

[0]: https://en.wikipedia.org/wiki/Economy_of_the_United_States#G...

[1]: https://www.wise-owl.com/investment-education/is-there-a-cor...