The word "mistake" is undefined... what is a 'mistake'?
If you mean mistakes of memory (i.e. I meant to spell frute correctly, i.e. fruit).
The truth of the matter is human minds have limited computational capacity, meaning they are both
1) slow
2) have not a lot of memory
3) a lot of our functioning is beyond our control (do you choose when to age? whether or not your blood vessels, wounds get healed, etc?)
...
If you mean, why do humans think so badly.. the truth is human beings live and think in their imagination, everything they think is ultimately imaginary in some sense and not 'real' it is real in an informational way (i.e. it the information exists), but whether that information is
1) Good
2) Bad
Is hard to figure out, then we have instincts (feelings) layered over top of our logic, and even sometimes are logic is false because we do not compute things mathematically in our heads, we take huge shortcuts due to time constraints.
We've all done it, thinking "If I do this, then logically this..." instead of listening to our 'gut' when time is short.