Guardian Angels are just terrible at their jobs. In fact, the average G.A. is a real slacker. I mean, so many calamities happen to people - you have to wonder what their angels were doing at the time.
Or, their apparent complacency might stem from fuzzy mission statements.
For example, let's say you're standing on the sidewalk, and a piano is falling from the 23rd floor, and it looks like curtains for you.
Since everyone knows that God doesn't make mistakes; there are no accidents; and everything happens for a reason... well, what's your Guardian Angel supposed to do?