I don't believe "someone else is going to help so why bother" is the real reason.
My candidate: social awkwardness and feelings of inadequacy. "Is this real? If I'm the only one making a fuss I'll look like an idiot. Oh please let someone else take the first step so it won't be everyone looking to me for direction. I don't know what I'm doing. Help, how do I CPR?" Often combined with a fear for personal safety. "Will the car explode? Will the bus fall on me? Is the drunk guy going to stab me if I touch him? Or infectious? Or about to vomit when I do rescue breaths? Will I go to jail if I get it wrong and he dies? Isn't it better to not involve myself?"
My candidate: social awkwardness and feelings of inadequacy. "Is this real? If I'm the only one making a fuss I'll look like an idiot. Oh please let someone else take the first step so it won't be everyone looking to me for direction. I don't know what I'm doing. Help, how do I CPR?" Often combined with a fear for personal safety. "Will the car explode? Will the bus fall on me? Is the drunk guy going to stab me if I touch him? Or infectious? Or about to vomit when I do rescue breaths? Will I go to jail if I get it wrong and he dies? Isn't it better to not involve myself?"