Why wasn't alchemy considered a real science?

It was at one time. It just no longer is.

A few thousand years ago, scientists did serious research and proved that the sun was a guy name Ra who drove his gold sports coupe (drawn by horses) across the sky every day.

A few hundred years ago, many educated scientists warned explorers not to sail too far from shore because they would fall off the edge of the world. Again, they had hard evidence backing them up.

A few years later, it was a proven fact that the world was indeed round. And it was round because god created it that way, right before he created the sun so we could have light.

Even more recently, the greatest scientific minds of their time all agreed that the earth was doomed by falling temperatures. Deforestation had caused our planet's surface to reflect too much sunlight, and we were all going to die in a premature ice age. They had mountains of peer-reviewed studies confirming this fact, and anyone who dared question them was branded as a denier. That was in the early 1980s, and many of those scientist are the ones screaming about global warming now.

That stuff wasn't mythology back in its day, it was hard scientific fact. The problem with science is that scientists can only work with what's in front of them. Forget having all the answers, most scientists are still looking for all the questions.