There were previous proposals like appcache which tried to do the right thing and handle a lot of details like this for the developer, but it didn't fit every u

There were previous proposals like appcache which tried to do the right thing and handle a lot of details like this for the developer, but it didn't fit every use-case well. So instead, browsers implemented something more raw that just focused on exposing all of the necessary capabilities. I think the hope is that web developers create reusable abstractions over service workers and most people just use libraries for dealing with them (and if any of the libraries end up on a really powerful abstraction, maybe that will get baked into browsers later).