Why do Americans like to blame on each other?

Your question seems to assume that all Americans have a habit of blaming each other for something. That is not accurate. There are people everywhere who blame others for their misfortunes or when they have made a mistake. People who do that do it for a variety of reasons.

In most cases it is a cowardly act or/and done out of some level of fear. The person doing the blaming is afraid to admit that they did something wrong. They may be afraid of reprisals or punishment. They may be so insecure about their position that they cannot admit they did something wrong. They may do it to hide what they have done. They may be afraid to admit they are capable of doing anything wrong. They may be afraid to tarnish their reputation or standing.

In any case, speaking up when you have made a mistake and asking what you can do to make up for it is always the best course of action and you will gain respect. People who are always blaming others become known to those they associate with and they eventually lose respect.