It's a combination of factors but it's mostly evolution converging on very similar systems across the plant and animal kingdom. Most neurotransmitters are relatively simple molecules while the ion channels on neurons are relatively complex. They have a high surface are for all those neurotransmitter, inhibitor, regulator, etc. molecules to bind to and the entire system is extremely sensitive to small changes.
Take cannabis, for example: the endocannabinoid system in our central and peripheral nervous systems effects a variety of things (memory, sleep, appetite, etc.) but it's also useful to plants, which evolved the system roughly 500 million years ago, independently of animals.
Take cannabis, for example: the endocannabinoid system in our central and peripheral nervous systems effects a variety of things (memory, sleep, appetite, etc.) but it's also useful to plants, which evolved the system roughly 500 million years ago, independently of animals.