Couple of things
1. Salads aren't cooked.
This is inherently dangerous, particularly in environments where fields may be fertilized with animal dung, and very particularly in environments where fields may be fertilized with human dung.
This is probably why salads don't feature in a traditional Chinese diet.
2. Vegetables (as opposed to fruit) generally have no evolutionary interest in being eaten, and will go to phytochemical extremes to avoid it.
The edible vegetables we have are a product of long artificial selection for palatability, but it seems doubtful if that selection has eliminated ALL toxins.
I'd bet there are still some lurkers with sub-clinical effects, which havn't been found, because no one has looked hard enough.