One year ago, U.S. stocks were just beginning to recover from the swiftest and deepest selloff in modern memory. Today, after a vertiginous rally, they are trading near all-time highs. Yet, the nationwide rollout of Covid-19 vaccines, the persistence of ultralow interest rates, and expectations for torrid economic growth have convinced America’s money managers that the stock market still has more room to rise.

That’s among the foremost findings of Barron’s spring Big Money poll of professional investors: 67% of the respondents...