The main reason these mushrooms are eaten is that they are misidentified as some similar looking edible species. The most frequent victims for these mushrooms are immigrants that mistake them for an edible species that they would find back where they were originally from. In the US on the west coast, that most often means immigrants from eastern Asia mistaking them for Volvariella, volavacea, commonly sold in supermarkets in cans as "Paddy Straw Mushrooms".
As far as being deadly, their lethality depends mostly on how much of them you eat. In a very general sense, if you eat some and don't seek medical treatment, your odds of dying are around 50%. With treatment (before the milk thistle extract), the survival rate was more like 90%.
There are lots of other mushrooms that also produce the same toxins in potentially deadly quantities. The ones that produce the most poisonings are Galerinas (especially G. marginata), since they resemble some of the hallucinogenic species of Psilocybe and can grow in the same habitats, at the same time, and even side by side with them. Lepiotas and Conocybes (Pholiotinas) can also be deadly in the same way, but don't generally resemble other mushrooms that most would want to eat.
There are lots of safe mushrooms and groups of mushrooms that are easy to identify accurately enough to eat without significant risk. Members of the genus Amanita (the ones these deadly ones belong to) don't fall into that category, unless you're a real expert. A lot of the "experts" that are referred to as such are people that can identify a few species (or maybe a few dozen species) in the woods - not somebody we should treat as a real expert. It's a bit like calling somebody who has done a "Hello World" program in a couple languages a programming expert.
If you want to learn enough to forage for your own wild mushrooms, you should contact a local mycological society. You can meet people who can show you how to identify some of the easier, safer mushrooms in your area.