Maybe horses ENJOY being dirty. Especially because it gets them so much scratching and grooming from those funny monkeys who hang out with them. "That was such a nice scratch! I think I'm going to roll again!"
I just bathed my ancient mare, and the ancient gelding was licking the water off her coat and pestering me to bathe him too. Should have taken photos. He came right up to the sponge and looked at me, "Come on!" -and scrunched up his eyes and moved even closer when I ran the sponge across his back. Here I was with a bucket of water and squished between two horses. It's been a hot day, so he probably liked the cooling as well as the attention. When the towel came out he virtually rubbed himself on it!
I don't have spotless horses unless I'm taking them somewhere special, but I do groom them a lot. I go easy on the shampoo as it strips out natural skin oils - often wash without, only shampoo when really dirty and then usually condition with something mild to put some oil back in. Brushing promotes distribution of skin oils through the coat, and the oils protects your horse's skin and hair.
Any of you have a horse that likes to play with the water coming out of a hose when being washed? My father has a stallion who constantly wants to grab the hose during washing. He drinks from it, blows bubbles with it, snorts into it, it's hilarious, and all of that with this innocent expression on his face. I should film him sometime.