I can build out a backend with a microframework and sqlite in no time. Why doesn't every company just throw out all the frameworks (or frameworks disguised as libraries) and just do what I did?

Because it doesn't scale for large projects and is impossible to work on with larger teams. Rails may seem bloated to some people, but if you are creating a set of complex business rules that constantly change and cost too much to rewrite with the framework flavor of the day, it makes a lot of sense.