A surplus of empty office space threatens to hollow out U.S. business districts. Don’t blame the pandemic.

America’s office glut has been decades in the making, real-estate investors, brokers and analysts say. U.S. developers built too many office towers, lured by federal tax breaks, low interest rates and inflated demand from unprofitable startups. At the same time, landlords largely failed to tear down or convert old, mostly vacant buildings to other uses.