Why does he do that?

I have a beautiful fluffy black cat (part Angora, part Manx I was told) who hardly ever goes outside, because he scares very easily. However, when he does go outside, the first thing does is roll around and get completely coated in dirt, sand, and/or pollen (depending on the season). He then comes back inside, carrying nearly a pound of dirt in his fur. Okay, I over exaggerate, but you get the picture. Why does he do this? Does he think I have nothing better to do than clean up after him? UGH! CATS!