One of new 3.1 features actually makes it easier for you to use external assets like gems and plugins.
Instead of downloading downloading code and then moving files to their proper locations, the new asset pipeline lets developers package such assets so bundler can accomplish that for you. You'll just specify what the asset is and what version you want in Gemfile.
That's a change that you can't just rely upon some gem to make. It's really the responsibility of the framework. And it creates order, making Rails easier to work with, especially when the app gets complicated.
Similar changes have similar rationales and these can be understood by watching videos like this one:
Instead of downloading downloading code and then moving files to their proper locations, the new asset pipeline lets developers package such assets so bundler can accomplish that for you. You'll just specify what the asset is and what version you want in Gemfile.
That's a change that you can't just rely upon some gem to make. It's really the responsibility of the framework. And it creates order, making Rails easier to work with, especially when the app gets complicated.
Similar changes have similar rationales and these can be understood by watching videos like this one: