There may be several reasons for this -- but I suspect they can all be summarized in one word: pride. Admitting we were wrong is a blow to our pride, but if we can blame someone else for what we did, then our pride remains intact (at least in our eyes).
But when we blame others and refuse to admit that we're responsible for what we did, we're only deceiving ourselves -- and that is a sin. But more than that, if we refuse to accept responsibility for the wrongs we've done -- no matter who we are or what we've done -- then we'll never turn to God for the forgiveness we need.
Do you remember Adam and Eve in the Bible? God had warned them not to eat the fruit of one particular tree -- but Satan told them that God had lied to them, and only good things would happen to them if they ate the forbidden fruit. They believed him -- but when God confronted them, they did exactly what the human race has done ever since: they refused to accept responsibility. Instead of admitting his sin, Adam blamed both God and Eve: "The woman you put here with me -- she gave me some fruit from the tree" (Genesis 3:12).
The real question you must face, however, is this: What will you do with the burden of guilt you now bear? Will you keep bearing it -- and even be proud of it? Don't make that mistake, but turn instead to Jesus for the forgiveness and new life you need.