>>718787559
The floor is dirty outside. That makes their feet dirty. Wearing shoes protects them and provides to keep clean water inside their slimy bodies. The cleaner they are the more shiny is their skin. Dirty slimes are seen as more primitive and uncultural race. In addition there rules are set due to hygiene of buildings that slimes should wear footwear in public.
Only places like beaches, parks are not under this strict rule. Slimes don't feel any kind of pain when they wear shoes and socks compared to humans.