Why don't they Mongoose ?
The reasons vary from individual to individual. For some it's because their past is just too distressing to face up to. I've recently done a fair bit of work with ex soldiers who have had PTSD from conflicts as far back as the Falklands in 82 or as recent as tours of Afghanistan. Some of their experiences have been nothing less than harrowing. the external support offered to them has been very poor and reliving those experiences has in many cases re-traumatised them as part of the healing process (Not to mention me, on more than one occasion). It's hard to volunteer for something that you know is really going to hurt.
The biggest reason by far is that many of us are conditioned not to show emotion. Think about the little lad out on his bike at the age of 6. He falls off his bike and scrapes his knee. Running into the house, all he or any kid wants is a big hug, a magic plaster and some reassurance that things will be ok. He needs time to let the shock of the incident and the injury fade. If instead, he gets a bollocking for crying, told not to be so soft and that big boys don't cry, he'll feel some confusion and a lot of shame. If this message that showing emotion is bad is consistently reinforced and he suffers shaming every time he cries, then that kid will stop outward displays of emotion. He''ll feel that it's wrong to display feelings and feel in fact, that emotion isn't helpful or useful. You'd be amazed how many guys I see who tell that story - usually after their partner has left with the kids, often with someone else who has more emotional intelligence - or a different history in respect of emotion.
The cancer thing is a little different. That's usually down to the hope that it will go away or a fear that it won't. If you haven't been diagnosed with the Big C, you haven't got it.
Human beings. We're incredibly complex things.