Optimize CSS for production using webpack 4

Question

I want to use OptimizeCSSAssetsPlugin for optimizing CSS in my react application. Tutorial here

https://webpack.js.org/plugins/mini-css-extract-plugin/#minimizing-for-production

The thing is that they add it in minimizer, which will override all other optimizations in production, including:

• FlagDependencyUsagePlugin
• FlagIncludedChunksPlugin
• ModuleConcatenationPlugin
• NoEmitOnErrorsPlugin
• OccurrenceOrderPlugin
• SideEffectsFlagPlugin
• UglifyJsPlugin

https://webpack.js.org/concepts/mode/

And when add OptimizeCSSAssetsPlugin to minimizer, they override all the default plugin and I notice that they also add UglifyJsPlugin to minimizer.

My questions is that why dont we add OptimizeCSSAssetsPlugin to plugins array instead of minimizer and be happy with other default plugins?