If a substance is abundant in the environment, you don’t need to spend your energy and reduce fitness in other areas to produce it on your own. Many species as a result might need things like certain amino acids found in their food that they don’t produce on their own, instead taking advantage of what is there. That logic shouldn’t really stop at nutrients or amino acids. There is no doubt an advantage to seeking out environmental compounds that act as neurotransmitters, saving you from having to spend energy generating those compounds internally.