It could be because the human body is a mysterious thing - sometimes you get itches and aches unknowingly and it could be 101 causes which is probably what you'll find when you google "knee pain" for example. Money is another issue as healthcare nowadays are not getting any cheaper and people tend to dismiss some occurrences of pain as an unknown once off encounter.
Another reason could stem from human psychology because all humans fear mortality. Everyone is brought up to understand that someday we all have to pass on and life is not forever. However that does not stop people from feeling that they are invincible and maybe, just maybe they'd be the one to beat the odds. Admitting to pain and illness often opens the door to mortality and people fear crossing the line to the other side where they might find situations of death knocking on their door.
Otherwise it could also be that people tend to fall into habit and grow complacent. This not only applies to health, wealth, dreams and anything in particular. Over time people who doesn't break out of their comfort zones tend to just stick to they do habitually and ignore other foreign things until something out of the blue hits them in the face. That's when realization dawns on them and they start paying attention - more often than not is when the situation gets bad.