Why col is stacking side by side

I am trying to stack column horizontally i.e side by side .
They are not stacking.

On the official website they have stacked column side by side : https://react-bootstrap.github.io/layout/grid/

Can anyone help me ??

Update : I solved the problem by adding import 'bootstrap/dist/css/bootstrap.min.css'; to index.js and adding bootstrap in dependency.

Can anyone tell me why it is necessary since i already added react-bootstrap as dependency ?