Why does W3Schools make a section just for bootstrap?
Why does it make it side by side with html, CSS, php, and MySQL?
Bootstrap is a framework, a css framework to be more precise. Developers are encouraged to know the underlying technology or language first before learning frameworks. hence the separation of bootstrap from css.
Also you might wanna know that bootstrap isn’t the only css framework out there, this is W3School’s way of not enforcing a particular technology on you.
Remember when you were in school, you were first taught A,B & C’s before learning words or tenses. learn the basics of the alphabet and you can grasp any language (definitely not Chinese 
)…you get the gist though.
Another CSS framework which helps you to build responsive grids is Zurb Foundation, by the way: http://foundation.zurb.com/
I actually like it more than Bootstrap.