This isn't exactly true, since the human meaning of "enhance" is quite different from the evolutionary meaning. For example, iodine (an element, not even as big as a molecule) supplementation at the fetus and early childhood stages can, among other benefits, result in as much as a 13 point IQ increase from a previously deficient region's average. This is a huge enhancement by human standards. Why hasn't evolution found a way to have our bodies produce iodine automatically (perhaps by producing iodomethane like some algae?) at that stage of life, or maybe only by the mother when the child is in the womb or perhaps produced and distributed only with breast milk? Then even if you lack natural sources of iodine you'll still benefit. It should be easy, it's just a single element!
Well even for "simple" things like the addition of a single new element, or a single new molecule, or a single new protein, the details are complicated, and any mutation that depends on a prior mutation to be beneficial has to wait until the prior mutation is close to fixation and is common among the species. Evolution is slow and only cares about reproductive fitness, and can easily get trapped in local maxima and a species can even evolve to extinction. Because of that, it can miss obvious (to human intelligence) low hanging fruit. So I don't think that "evolution would have found it already" argument is very compelling. Evolution is dumb, it's amazing it works at all.