Observation: If Jerry was truly into hygiene he'd never allow people to put their shoed feet on his couch, which happens in many episodes.
City sidewalks and floors are a repository for all kinds of crud brought down by gravity, covering shoe-soles with a film of spit, urine, grease, grime and unknown chemicals. Its presence may not always be obvious but just think about where you walk in a normal day. People almost never clean the bottoms of their shoes.
If you wear shoes inside your house (which I do for convenience) it's wise to keep them on the floor and keep everything else off the floor unless you plan to wash soon. I wouldn't sit on steps for the same reason, especially in an old apartment building with heavy traffic.
On the other hand, being out in nature seems fairly clean to me. Dirt and sand clean your shoes as you walk and you're usually away from industrial filth. The outdoors seems mostly self-cleaning unless you step on a pile or it's too muddy.
Alturas