I'm 38 and spent my whole life trying to answer the same question.
Why do I stub my toe? :(
Why do I spend my paycheque on beer and cigars? :(
Why do I hang my queen? :(
Life and chess are full of mistakes -- that's what makes us human. I go into a game as prepared as I can be, but at some point you just have to lean on destiny. I have no reasonable answer aside from that.
Do you have some check system for preventing blunders ?
When I realize that I made a huge blunder, I feel tempted to pound my head more than 100 times...