So why didn't they use sheet metal? Were the engines too weak to pull a metal covered craft through the air?

Pretty much this. The most powerful WWI aero engines tended to deliver between 100 and 200 HP, which was not enough power to propel heavier machines at the speeds and maneuverability demanded by air combat. There were other limitations for aircraft designers in this period besides engines. For one thing, the mass production of aluminum and aluminum alloys was still a relatively new technology. Some design firms did incorporate duralumin alloys in their aircraft which allowed for a stronger airframe without the weight penalty that came from using other metals. The Junkers firm was one of the early adapters of this technology, with all-metal aircraft like the D. 1. But Junkers was something of an exceptional firm and most of their aircraft were conceived as ground-attack aircraft or observation craft. Metal or armored aircraft did not have the performance to serve as frontline fighters or scout craft. Moreover, multiple wings was a means to gain lift and maneuverability with less of a weight penalty. Far from being a problem associated with weak materials, the multi-plane wings of WW I aircraft were a reasonable response to a problem of how to wring out the best performance from the engines of the period.