Why do hackers believe they will not be caught?

Because the vast majority of them don't get caught.

For every high profile case of a hacker being caught and brought to trial there are tens of thousands who just get on with it, and either don't raise enough suspicion to be caught, or don't create enough damage for it to be worthwhile for the authorities persuing them.

You mind as well ask why thieves believe they will not get caught. Really, for an average hacker or thief in my context, there's a good chance they're left free.

If they're not making such a high profile of themselves then they're likely not going to get caught. Which kind hacker are we talking about? The one that actually exploit computer systems? Make cracks for paid software?

Unless they're making such a ruckus such as to steal money by exploiting certain things (which will be a big deal), then they have little to worry about. "Hacking" can be very broad. Heck, not everything can be traced back to the point of origin or even cared for in the first place. I doubt thieves have to watch their back if they stole a pen.