It's that they're skinny. Obesity causes obstructions and that's why the vast majority of obese people have sleep apnea, and the vast majority of apnea sufferers are obese.

But, since non-apnea related snoring often happens because of things like sickness/allergies, excessive drinking, and smoking, I promise you there are homeless who snore.

Having slept with several dozen homeless people I can state unequivocally that they DO snore, but only after a big meal. This is why I try not to feed them too much before I kick them out.

No, but the guy on my porch talks to himself all night.