In the US there’s also deep religious influence that paints suffering as virtue that’s been exploited by capitalists to establish a norm of pushing through the pain and continuing to work instead of taking a break to figure out what the root cause is and address it. Some also just don’t have a choice, because taking time off means not getting paid which means losing a roof over their head.
2. They may be so used to pain they don't think to mention it.
3. A lot of people lie to their doctors for one reason or another.