Rails is an open source web development framework that excels in productivity by coming with smart defaults for people to use to get up and running quickly.

Rails already integrates with webpack, and you can certainly implement your own integrations to further advance things - and you can turn around and make this a gem for others to use, or contribute it to core if its something more fundamental.

But for the most part, the people behind RoR are pretty happy using RoR for their development, and don't need to use React or Vue for a majority of what they do.