They did it for control and convenience at the expense of literate computer users.
The Windows 10 UI is written in TypeScript, Edge is chrome, Chrome is chrome, Safari is chrome - it makes hiring and training easy when you can just make every API a grey gelatinous chunk of ECMA script with hamburger menus. It also means the "app" is always sandboxed unable to do powerful things like universal push-to-talk.
The color and joy of using programs has been flattened out in the name of consistency.