Commentary A couple of weeks ago, the buzz on Wall Street centered on a new stock offering. The heretofore private electric car startup, Rivian, listed its shares on the NASDAQ exchange. This initial public offering (IPO), as such things are called, was the biggest since Alibaba’s 2014 listing. Investors certainly have an appetite for electric vehicle manufacturers, a fact to which the sky-high valuation of Tesla testifies. But as the patterns of the last few months have made clear, that appetite is broader based. This year has seen a flood of private companies list on exchanges for the first time, most met with enthusiasm. This activity would seem to carry two seemingly contradictory signals about the future. On the one hand, the surge of IPOs speaks to business optimism about the economy by both the listing companies and investors. On the other hand, it signals a belief among managements that …