Because that's always been the mission. Have a solution for most people, most of the time. With the target being building full-fledged apps ala Basecamp (that touches on all parts of web tech).
So we've been pursuing this mission for 7+ years. We've just gotten better at it.
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:
Why? Why is Rails changing to handle this, instead of add-ons, plug-ins, and gems changing?