The leading theory at the time was that metals were grown in the earth, starting as base metals and transmuting over time/under certain conditions into the higher metals, eventually ending up at gold, which they thought was the end point because it never tarnished. It was actually not a terrible theory given the information they had, all metals come from the ground after all - the idea of turning lead into gold wasn't some magical thinking, they were trying to reproduce natural conditions in the lab and speed it up, just like we do today in hundreds of other ways today. If someone had succeeded it would have been like doing the double slit experiment of it's day, a complete proof that alchemical theory was right.