Because we call them mistakes instead of challenges that we can learn valuable lessons from. What would you rather, make a mistake? or make a challenge that can teach you many valuable lessons?
I think part of the problem is that we assume fault with mistakes. If someone makes a mistake they can easily be treated like they were stupid or lazy or whatever and that caused the mistake. When this happens it is easy to try to get out of being blamed instead of learning from it and then getting over it. I see this sort of blame junk often, something bad happens, a mistake or not (could be a accident that could never have been prevented) and the first thing everyone in charge seems to do is look for someone to blame. Whose fault is it? Not, how do we recover from this?
Sh*t happens but as a group we seem unable to fully accept this or understand it even.