If a trait would be advantageous to an organism, why hasn't it evolved?

Because to actually use the word evolve an organism must suffice more than one condition, which in this case would be an advantageous trait. Having an advantageous trait (from our eyes at least) must also include the right and stimulating enviironment for procreation. So this answer is multifolded.

Evolution happens because it can't not happen. Evolutionary change is a property of life.