Every body knows about traffic rules but why many people disobey the rules?

You said everybody knows about traffic rules. I do not think so. Some section of the society are not aware of traffic rules. Secondly some people are in such a hurry (unnecessarily) so they would like to violate the traffic rule and proceed. Some people are habitual trafffic violators.

Lack of self discipline, Selfishness, lack of concern about the society and fellow beings and above all they are able to get away with the crime. Punishment need to be stringent and people with basic qualification need to be given licence. If the violation is repeated the authorities should cancel the licence for ever. these few measures may reduce the violations but may not avoid fully.

I agree with both of the previous respondants. I think, at least in the U.S., there a bit of "me first". And, the likelihood that they are going to get caught is rather slim. And, if they did, it's not a big punishment for many people. I also agree that a lot of people don't know all the rules. I've been berated on the road because I was following the rules, but someone thought I wasn't and got mad. I took a traffic class with a traffic cop and learned many things that most people either don't know or thought was wrong.