Why @babel/plugin-proposal-class-properties is not supported by default?

[Morritz]

Support for the experimental syntax 'classProperties' isn't currently enabled.
The solution was to use: addExternalBabelPlugin('@babel/plugin-proposal-class-properties') in config-overrides.js along with addExternalBabelPlugin('@babel/plugin-transform-react-jsx'), because putting "plugin-proposal-class-properties" in babel.config.js didn't make effect.

Something should be done that it would be supported right out of the box.