I couldn’t help but notice that 80% of the people sat there looked perfectly fine, chatting etc and didn’t appear to be in either an accident or emergency.
You are being v v unreasonable. You have no idea at all what's wrong with people. My late father, when he was admitted to hospital for the last time, would have fit your criteria above. In fact, he had a soon-to-turn-dangerous fecal impaction due to his secondary terminal lung cancer, which had spread to his bones. He died three weeks later. Yet he walked into A&E that day, sent there moaning and groaning about the fuss, because he had diarrhea which can signal a potentially serious side effect of his immunotherapy, so anyone with that profile needs to be seen asap.
Likewise my mother seemed TOTALLY FINE to the naked eye when she had terminal cancer, except for the last three weeks. But people in that situation have a million things going on in their bodies which can take them down pretty fast.
Another example is AIDS, where the virus hides in "pockets" all over your body and, if you can't get your meds for some reason, or something else is going on, those "pockets" are the reason you can get very sick, very quickly. They all activate at once. But in the early stages of that happening, a casual observer wouldn't know.
ETA: I went to A&E with chest pain, had an initial EKG which was fine, but then I had to wait for a lung scan because they were concerned I had a blood clot in my chest from a long flight. Which I did, but I still looked normal. You wouldn't have known I had chest pain to look at me.
It's actually incredible what serious trouble people can be in and look OK - before getting worse.