Not true at all, at least for Ruby.
Most larger React projects have _far more_ dependencies_ than an equivalent Ruby project. You could chalk it up to a few things I think:
* the JS ecosystem is just much larger - more devs (especially inexperienced ones) means more code and more copy/pasting solutions that rely on `npm install` from the web.
* JS has just not been as stable and mature as Ruby or Python until the past couple of years (thanks to ES5/ES6), especially if you consider the browser acceptance of latest ES.
* The general explosion of NPM packages that do dumb, trivial shit - see https://www.npmjs.com/package/is-odd and https://www.davidhaney.io/npm-left-pad-have-we-forgotten-how... for two examples.
* There are at least four major tools to manage dependencies in JS: npm, pnpm, yarn, and yarn 2. This often means you end up with multiple ways teams are handling JS deps in their projects, with all the non-essential complexity that comes with it. In Ruby there is just one way - Bundler. I don't even want to talk about python.
* There are more leading frameworks to choose from in JS, which means you have libraries that support all of them, doing things in slightly different ways. Consider a legacy Angular app that updated to React in 2018 and is updating to Svelte or whatever now. If the team hasn't been very disciplined in updating and removing all transitive dependencies through the upgrades, you will inevitably end up with a ton of stuff (is `react-date-wrangler-jobber` used? I still see references to it, but is that page live? who knows!?) in your bundle hanging around and cluttering up your build.
I've been updating things across our main React/Rails product at $current_job. Most major dependencies were out of date by 1-2 years. Updating Ruby has been *so much* easier, whereas the React / npm stuff often ends up in a rats nest of conflicting dependencies. And this is after we made a major effort in our build and packaging setup across the board.
I do think it is getting better in the npm world, slowly. Ever so slowly.