Comment by ACCount37

Natural selection cuts both ways.

Sure, many things evolved to be less edible. But humans themselves are hunter-gatherer omnivores - who evolved to be very good at eating a lot of very different things. There are adaptations in play on both ends.

There are, in fact, many countermeasures that would deter other animals, but fail to deter humans. In part due to some liver adaptations, in part due to sheer body mass, and in part due to human-specific tricks like using heat to cook food.

If your countermeasures just so happen to get denaturated by being heated to 75C, good luck getting humans with them. It's why a lot of grains or legumes are edible once cooked but inedible raw. The same is true for many "mildly poisonous" mushrooms - they lose their toxicity if cooked properly.

Those countermeasures don't have to be lethal to deter consumption! If something causes pain, diarrhea or indigestion, or some weirder effects, or just can't be spotted or reached easily, that can work well enough. So the evolutionary pressure to always go for highly lethal defenses isn't there. It's just one pathway to take, out of many, and evolution will roll with whatever happens to work best at the moment.

Human takeover of the biosphere is a recent event too, and humans are still an out-of-distribution threat to a lot of things. So you get all of those weird situations - where sometimes, humans just blast through natural defenses without even realizing they're there, and sometimes, the defenses work but don't work very well because they evolved to counter something that's not a human, and sometimes, the defenses don't exist at all because the plant's environment never pressured it to deter consumption by large mammals at all.

And with the level of control humans attained over nature now? The ongoing selection pressure is often shaped less like "how to deter humans" and more like "how to attract humans", because humans will go out of their way to preserve and spread things they happen to like.