why is It that almost no site uses appcache, when it is a way of ensuring that resources are already local and the only network traffic is the truly essential p

why is It that almost no site uses appcache, when it is a way of ensuring that resources are already local and the only network traffic is the truly essential part? HTTP caching is fine for that. appcache is more about allowing the site to largely work even after going offline. That's more than most sites need.