Firms can now privately raise hundreds of millions of dollars privately.
Because we've been in an incredible bull market for more than a decade, interest rates are close to nil, confidence is high and money is plenty.
Once the next bust cycle begins -- ie. the "beginning of the end for the stockmarket" -- even large scale investors will lick their wounds, leading to a decrease in private investment. Which in turn will many companies see requiring public investment again.
Also, since most new startups are digital in nature, they just don't require as much backing. Software can be copied at almost zero cost.
What modern startups can save in terms of infrastructure and distribution, they need to make up in terms of expertise. Software development ain't cheap. Scaling a business isn't either, even in the digital realm.