This bridge was just fine for five months. Then it wasn't.
Dinosaurs roamed the earth for 135 million years. (By contrast humans have been here for a million or so.) Then they didn't.
etc.
See why your argument doesn't quite work?
Not really. I was under the impression some like to hack and attack coins for sport or to prove a point or to learn. GingerAle's answer seems to be the correct answer to me : the flaws are only exposable with enough resources.
Try to put yourself in the position of a malicious attacker. If you have an exploit that you can use repeatedly for personal or financial gain, how quickly would you be willing to announce your discovery?
With that in mind, how important is it for a system that deals with other people's money to be cryptographically secure?
I don't know for sure but I don't think that's actually it. A problem with holding an exploit in your pocket is that someone else might use it or it might get fixed.
Mostly I think people just don't care about DRK very much. People are constantly trying to find exploits for Window, Linux, Android, etc. Maybe even Bitcoin. If anyone at all is trying to find exploit for DRK it can't be very many people, nor are they likely putting much time or effort into it.
There is a huge amount of self-importance in these coin communities where people think their tiny little project matters very much just because it is a "top 10" coin or whatever. In the bigger picture they really don't.
EDIT: Oh I misunderstood JM's point. I think he was saying that even if it were being exploited, you wouldn't know about it. I agree. That is likely true of most active exploits.