I may be more alert to such incidents.
I'm an ex nurse and have ended up volunteering being stuck with being the first aider in almost all my non nursing workplaces AND voluntary roles too!
But yea, shock, bystander effect, fight flight & freeze. I think a lot of people think - someone else will step in or that they'll do something wrong. Sadly I also think (especially where I live and yes there are big drug and alcohol issues here) that what is actually a hypo/stroke/faint is someone drunk/high and they're either judgmental or fearful of getting involved (there has been incidents of well meaning people trying to help someone getting stabbed - inc with syringes which IS terrifying).
Training can have some effect but even with training some people still freeze or panic.
I've dealt with (outside of work) ...
Several asthma attacks
Allergy attacks inc anaphylaxis
1 suspected heart attack
Strokes
Hypos
A fellow student taking a SPECTACULAR tumble down EVERY step of a lecture theatre - fainted due to flu
A few fender benders
Burns and scalds - no of people I've had to say no you don't put butter, Vaseline, coconut oil straight on a burn 🙄 we seriously need good quality first aid psa's!
A motorcyclist hit by a speeding ARSE of a boy racer - wiped the biker right out - example of shock making people weird - his ankle was clearly broken, I think he busted his clavicle too - what does he bemoan? His ripped leather trousers, expensive he'd just bought them. This incident also provides a reason why people are reluctant to help, another person had rushed over to help him and tried to remove his helmet when there was blood around his neck and he'd clearly injured that area - big no-no! I actually screamed "no!" As I ran across the road and the guy helping caught himself and went "shit! I forgot" as I got nearer he said "thanks, I'm first aid trained but COMPLETELY forgot about not removing helmets with a suspected spinal injury"
I clearly need to work on resting bitch face too as I'm ALWAYS the person asked the time/directions/when does X shop shut - this has happened in places where I'm a visitor too and I haven't a clue - and they don't believe me!
I've also called 999 for fire and police for a few kitchen fires (neighbours headed straight for mine), dv, a robbery in progress (THAT was scary - I was in a phone box nearby, dark but not late in the days before mobiles when I was dating my ex and it was a ldr. Was -about to phone to him, next thing I know 4 guys breaking into a jewellers not 3 doors away! If I'd left the phone booth they'd have known I saw them. Turned my back to them, hood up, called 999. Police came, yanked me out the phone booth into a car. Arrested them. I don't mind admitting I was snotty crying and in a state! That was scary.
Helena 
What a twat! Actually what a bunch of twats! some fucking footie fans Called police? Though it looks like he needed an ambulance after being a dick! Tempting to treat him sans anaesthesia - if he even needed any. Hope the driver was ok.
"In all of my 40-odd years, I've never encountered any kind of emergency." Really never? Never broken a bone or sprained an ankle or had a sibling do so? (Admittedly my 2 siblings were fucking bonkers so no wonder we spent I reckon the equivalent of at least 2 summer holidays on a&e's!)
Every incident has happened nowhere busier than a small town, though I've lived in 3 cities
On a thread a while back I admitted I'm calm in a crisis - then fall apart later. It's a weird thing. Like you go on pause!
I think me op and others must be getting other people's bloody shares of trauma witnessing 
Anyone is welcome to try suing me! I ain't got a pot to piss in! 
Yes risk assessment - inc risk to yourself is also important. Can't help anyone if get hit by a car running to someone else's aid eh?