There must be a reason, though. If it were that easy to increase your brainpower that way, and there were no other drawbacks, this modification would already have been selected for.

If you take them, perhaps you will discover later why this potential evolutionary change was not beneficial, and not selected.

Maybe the ability to synthesize a particular chemical could not (or merely has not yet in humans) be reached by a single change, or a path of individually selected-for single changes. In other words, your argument only works if the reachability of the improvement is high enough, and our species has existed for enough time in an environment where it would be rewarded.