The cars in Project CARS should look dirtier.
They can get very damaged and dirty. Check out a few of these shots that other people in the community have taken: http://imgur.com/a/ULQeq#46
Some of the cars don't have rain or dirt build up enabled yet tho but eventually they all will.
Here's one with some damage that I've taken:

pCARS 2013-04-09 01-15-37-73ss by darkdeus, on Flickr