Consider this scenario: you've pulled into a rest stop along the highway to use the bathroom and buy a bag of Doritos. Despite having washed your hands after relieving yourself, you've still touched the door handle, which is germ city. On the way back to your car, you tear open the bag of Doritos and start munching. You get in the car and put on your seatbelt, because you always wear one. Good for you!
I bet you don't even need to hear the rest of the scenario, but here it is: all those germs from the public bathroom plus all that Doritos dust was just transferred to the seatbelt. Multiply that by all the times you eat in the car or don't have the cleanest hands in the world and you've got one hell of a filthy seatbelt.