Other threats to the new dominance of public markets loom. A handful of IPOs, including aesthetics device maker Candela Medical Inc. and NordicTrack maker iFit Health & Fitness Inc., were postponed in October, and regulators from the Securities and Exchange Commission are starting to question the optimistic revenue projections used by startups that are merging with SPACs. An SEC warning that could require some SPACs to restate their financial results put the brakes on some new offerings.

There are some public companies going private, too. This week mattress-in-a-box maker Casper Sleep agreed to be taken private in a transaction that values Casper at less than $300 million, about half what it was worth when it went public nearly two years ago. Casper had been valued at $1.1 billion in a private funding round in early 2019.

But investors, bankers, and others close to the IPO market say the frenzy for public companies isn't over yet. For fund managers who pick stocks, buying IPOs is still a way they can outperform the broader market and differentiate themselves from index-tracking rivals, according to analysts. For companies, providing more cash to employees in the form of stock is a way to retain talent during a time of labor shortages. And for bankers, the IPOs represent huge fees. Rivian, for example, said it will pay up to $195 million in underwriting fees to banks for its IPO, according to a regulatory filing.

"When you get invited to the business Super Bowl, you don't say no," said ZoomInfo Technologies Inc. CEO Henry Schuck, who went public last year. "It's cool to be public."

Write to Corrie Driebusch at corrie.driebusch@wsj.com