Self-inflating PFDs are very good, and getting better. But like all manmade mechanical things, they can fail. They can fail to inflate or the inflation bladders can leak.

If you rely on a PFD that you have to manually blow up, then it is not much use if you go in the drink because you have been rendered unconscious, physically disabled [heart attack, stroke, hypothermia, whatever], or mentally / emotionally disabled [panic, dementia, too young to understand, whatever].

Chunks of closed-cell floatation material in a PFD float all the time, every time, regardless of your condition.