Exactly - the receptors were there because your body naturally makes endogenous cannabinoids. Those receptors are not in your body because of cannabis - They are there because EVERYONE's bodies make endocannabinoids. Cannabis and Endocannabinoids are completely independent of each other. The fact that cannabis works with your endocannabinoid system is the reason that it has effects on you.
And yes, people did just tried shit way back when. Some out of necessity (hunger), some out of curiosity (just watch a kid), some out of a search for medicine (they didn't have Advil - and chewing on a Coca leaf gets rid of that headache), some just on accident (using cannabis for kindling and getting high off the smoke). If it satisfied their hunger and didn't make them sick they ate it again, if it made them feel good they ate it again. As you know there are certain people that will smoke ANYTHING. On this site alone I have read first-hand-accounts of people that will literally smoke anything that burns. That personality type was surely present in humans 2000 years ago.
Look around outside. None of it makes sense. The only common denominator is that everything in nature is consuming something. All characteristics are for survival.
I don't pretend to have the answers. I surmise it goes back to genetic mutation and evolution.
I know the plant didn't "decide" to grow cannabinoids to please the humans.
You give the system too much credit. Your body has an Opiod Receptor. It's there for endongenous opiods
dynorphins,
enkephalins,
endorphins,
endomorphins and
nociceptin. It is not there for Opium, Heroin, Morpheine, Codeine - but they bind to those receptors and "trick" them by being similar enough to the endoopiods.
The same thing happens with shrooms and the 5-HT2 receptor. That receptor is for endogenous serotonin. Shrooms "trick" the brain into thinking there is a MASSIVE influx of serotonin. Psilocybin metabolizes into psilocin which has a similar structure as serotonin; a natural neuro-transmitter affecting our mood. The psilocin is then able to bind to serotonin's receptors, exciting nerves as if a massive rush of real serotonin had been released. The floodgates of the senses are open and enormous amounts of information comes in. This fungus didn't "decide" this so we would harvest them.
The same thing happens with cannabis. The body makes cannabinoids - cannabis has its own seperate cannabinoids - THEY ARE NOT THE SAME CANNABINOIDS. Cannabis's canabinoids are close enough to the body's so that it can utilize them.
It is not a elegant; they just happen to be close enough to bind to the receptors and trigger a natural response.