Answer:
Because it was not based on evidence and experiment.
Why isn't alchemy considered a science?
Alchemy isn’t rigorous enough.
People began to realize that it was possible to transform one material into another in antiquity. The study of how to achieve this was known as alchemy. Alchemists would experiment on various substances, trying to turn lead into gold or to create the elixir of life.
They were unaware of what was taking place in the background, though. They weren't as rigorous as contemporary science. seldom ever exchanged their discoveries. And technically speaking, they carried out their experiments before the formalization of the scientific process.
In a way that made perfect sense to the alchemists but makes no sense at all to us, alchemy blurs the lines between empirical observation and experimentation, irrational conjecture, and mysticism. The last two render it unscientific.
Alchemy became known as chemistry once the scientific approach was established and mysticism was eliminated. Because alchemy particularly refers to the earlier, less scientific kind of chemistry, we no longer view it as a science.
