The face is where all the mucous membranes are located (tear ducts, nasal passages, etc.). These are weak spots through which pathogens are more likely to successfully infect the body.
It's way thinner and full of holes. It's not so much about the skin on your cheeks, but your mouth, eyes, nose membranes. And even if you just dirty your cheek, the contamination might spread elsewhere with further touching. Think about this: How do you take drugs? Eye dropplets, oral blotters, snorting.