Because people form their first impressions by appearances.
If a vendor or potential employee pulls up in a slovenly car, the mental impression that I form is that this person is not concerned about appearances or results, is lazy, disorganized, and unable to manage his time efficiently. It may not be a fair assesment of this person, but it will be the first impression formed.
In addition, I simply feel better driving around in a clean car. Some people feel better in new clothes, I feel better in a new-looking car.
Simply my $0.02.