Silicon Valley threw a hot robot party, and the clankers were M.I.A.

At a sold-out conference in San Francisco for people who build robots, the robots were surprisingly hard to find.

This week, 1,500 robotics investors, founders, and engineers packed a cavernous former Army warehouse at Fort Mason, overlooking San Francisco Bay and the Golden Gate Bridge. They came from across the country for Actuate, a conference that bills itself as a middle ground between an academic gathering and a trade show.

A few years ago, a crowd that size would have been hard to imagine. For much of the past two decades, robotics was a difficult sell in Silicon Valley. Now it’s one of tech’s hottest bets, as investors look beyond…