why do smart people fail ?
I think the reason is simply that everyone and every type of person fails.
If you think there is some trend though, I would say it is overconfidence. Being smart doesn't guarantee you anything. People that aren't particularly smart recognize that they are going to have to work harder and so they do. They work harder and they are often successful. If you're overconfident in your own abilities then the following will happen:
a) you'll never seek help from others
b) you will put all of your eggs in one basket, thinking that one skill is all you need (it usually isn't)
c) you'll neglect your weaknesses
I feel like I'm a fairly smart person, but at the same time I fear failure. I know that there are no guarantees. Maybe this is pessimistic, I don't know.
Why do smart people do dumb things? etc.
I do lots of dumb things. Sometimes it's just for the sake of doing something dumb. Other times, it's because I based my decision making on emotion rather than intellect.