Because from what I've heard some job interviews it is not required shirt and ties.
I remember a guy telling me, the company he works at he was employing people, and one guy asked what the dress code was, and the employer said casual, so on the day of the interview, this guy turned up in very casual stuff and the other guy in a suite.
The guy dressed very casually got the job because the interviewers where more interested in what the guy knows rather than what he is wearing and apparently the interviewer also was more positive in regards to the casual dressed guy because he asked what they wanted them to wear.