The more complicated version of this is "if drugs were an improvement to our overall evolutionary fitness, we'd have evolved to do what the drug does by default". This opens up interesting avenues of thought, because some drugs aren't an improvement to fitness in the ancestral environment.

Stimulants usually make you burn more energy, so they're fertile ground now that calories are cheap. Dietary energy has only been affordable for a couple of centuries, so there's not a lot of time to have evolved more wasteful bodies and minds.