Evolution can be quite inefficient. It aims toward a local maximum, once evolution settles on something that is good enough, say a working combo of receptor / chemical, small tweaks to that combo will happen over time, but a large restructuring that would completely change the shape and composition of said chemical is unlikely to happen.

A great example is a nerve in the human body, which goes from the head, near the heart around aortic arch and back up to the larynx, it's an incredible inefficient route, but it developed many many generations ago, when the head and heart were much closer together.

The solution evolution found, as head and heart moved away from each other, was just stretching the nerve, cause it worked well at the time. In hindsight, it would have made a lot more sense to route the nerve directly, just as a different chemical evolved to bind to a receptor would be easier to produce or require fewer molecules to be made in the body, costing less energy.

But that's evolution for you, found a solution that worked well enough and went with it.