Bootstrap was just an example. It could Bootstrap, Materialize, W3.CSS, etc.
The point is that it's much more convenient to reuse code from a framework, because it's better to sacrifice file size for fast iteration and functionality.
There is always a tipping point. We tend to use Bootstrap a lot at work for the reasons you mention, but you can pretty quickly get to the point where your CSS is complex enough that you would have been better off doing it from scratch. All frameworks are like that -- you trade off initial convenience for design constraints.
When I'm doing my own projects I always write my CSS by hand because it ends up less complex in the end. I don't need to see pretty things up front like my corporate customers do.
The point is that it's much more convenient to reuse code from a framework, because it's better to sacrifice file size for fast iteration and functionality.