Great fan of Rails but man, that’s a lot of bloat. Last time I used Rails it only came with jQuery. Live-reloading was an exotic thing, quite difficult to setup. Now it seems to be enabled by default, explaining some of the stuff in there.
It's also 99% server-side preprocessors. Only a single package in that list (lodash) is something that could be actually loaded via browser, and even that would only be a small percentage of the full package size. It's also not required for a new rails project, it's simply a convenience workflow that integrates many of the tools used in modern best practices.
Your point is valid that a large portion is server side only (which is still a concern for my use case). But note a lot is omitted from my gist, there are 768 packages.