But, also, they were literally trying to turn lead into gold.

(The term 'alchemy' covers a broad swath of various people in various times, with various beliefs and practices ranging in the spiritual, metaphysical, psuedo-scientific, and scientific realms. For some, yes, turning lead into gold was pure allegory. For others, it was an actual literal goal and they hoped to become rich that way. And for some, it was both at the same time -- pursuing both spiritual enlightenment and economic enrichment through the exact same process. It's further complicated by the fact that many of them were persecuted in their time and/or feared non-alchemists stealing the recipes and techniques they had discovered, so even when they were writing down actual legitimate scientific findings and real chemistry, they often did so in fantastical, allegorical terms that only someone steeped in alchemical lore might be able to decipher.)

Edit:

You also have to understand it in the context of the time, before modern chemistry. It was possible to turn iron into steel, to turn copper and tin into bronze, etc. They didn't know about 'elements' as we know them today or atoms or how an element is defined by the number of protons in the nucleus. For all they knew, maybe gold was an alloy of multiple base metals ... and if it was, lead was a likely candidate for one of the ingredients, since it's also very dense and soft, much like gold. Maybe all metals were alloys of certain base ingredients, and the only thing that made one metal distinct from another was the ratio of those ingredients. If that was the case, then it should be possible to discover the exact correct ratios to produce gold, and/or a way to alter those ratios in an existing metal to turn it into gold.

Sometimes, even in modern chemistry, you can do reactions that look like straight-up magic, even to knowledgeable eyes. For someone experimenting in a primitive lab, seeing some of these reactions happening ... of course some of them associated it with actual magic and spirituality. And even those who didn't ... they often used the language of magic and spirituality to describe what they were seeing and/or to obfuscate it and make it less intelligible to outsiders they wanted to hide their knowledge from.