This is because extensions use a package manager to be installed and versioned and you don’t want those extra tools to be installed in your web root as they are needed only for development and the final files will be copied as needed.
So usually a lot of development tools are installed in your project folder that you only need during development and not on your live site. Tools like minifiers and compilers.
So we will be also changing the default for new Php projects to have them always in a sub folder and no longer in the root of your project.