This has been puzzling me for a long time now. There is an evolutionary predilection for living things which have evolved strong defensive measures (that may not themselves be visible) to give distinct visual or audible warnings advertising these measures.
Wasps, hornets, snakes, and bees are all brightly coloured. Snakes may even have rattles to advertise their venom. These things by in large don't want to have to use their poison in a defensive capacity. Getting revenge on a predator who has mortally wounded you is a Pyrrhic victory.
These warnings are so potent that many animals without such measures will mimic these warnings in order to piggy pack on a general evolutionary bias towards an instinctive avoidance of anything that warns of being poisonous.
But take the death cap mushroom.
This innocuous fungus looks identical to completely harmless mushrooms. It has no foul smell. In fact, evidence shows that it actually has a pleasant taste! That picture above actually looks pretty tasty in my opinion.
This fungus has gone to the trouble of developing one of the most potent poisons in nature (humans have a seriously robust digestive systems by animal standards, and yet a single death cap can easily kill an adult human).
So why so secretive about its arsenal?
I know this same question about evolving poisons that are not well advertised could be applied to a large number of animals, insects, and particularly fungi, but I think the death cap is the most extreme example in both the lethality of the poison considered, and combined lack of advertising.