They don't necessarily have to all out fail. Many get extremely noisy when the guides and tensioners get all chewed up, (which are plastic), to the point people bring in their cars for it. Then, the only solution is a replacement. Which is actually better. When timing chains fail on an interference engine, (which most are today), it completely trashes the engine, requiring a complete engine replacement.
I'm not saying it hasn't happened, but I have never heard of an engine getting trashed due to a timing belt failure.