I would state this as "chemical space is so large it's surprising that an evolutionarily distance biosynthetic pathway would, through evolution, actually find and optimize molecules that were remarkably effective modulators of an extremely distantly related organism." (I'm not ascribing intent or desire to evolution here, when I say "find and optimize").

But like many things in biology, if you view it through a billion-year evolutionary lens, it's not completely surprising. I doubt anybody could come up with a general rule or law that explained this well, and anything I did would just speculative arm-waving, of which this site has enough.