Science

Evolution doesn't select for positive traits, just not-negative. If a trait doesn't strongly reduce the chance of reproducing, it can get passed down.

For example, humans have plenty of neutral traits (hair color, eye color), and even plenty of negative ones (Alzheimer's, arthritis, baldness, cancer, sickle-cell disease). But they're not so fatal that they don't get passed down.

Similarly, if neutral traits like cannabis including whatever chemical causes the munchies doesn't reduce that plant's ability to reproduce, it'll get passed down.