For privacy reasons, resources are no longer cached across origins, so the size does matter (though arguably 1KB more or less makes absolutely no difference on any webpage that has a medium-sized image, especially if you have SSR for the first paint).
But the better reason why these projects exist is for experimentation and iteration. It's not like everyone who uses React will switch to this over the holidays, but maybe in five years some of the ideas in here will become more mainstream, and a future version of React (or a different framework) may adopt them.
Because it's a different threat model: Separating caches per-origin prevents a site A from seeing what resources you requested on site B.
But something like Decentraleyes prevents site A from seeing what resources you requested on site A.
(Or rather, whichever CDN provider site A is using.)
You could have both at the same time, but they are orthogonal. As for why it's not in browsers, assuming good intent, I'd think it's because it requires you to bundle a whole lot of libraries with the browser for it to be useful as a local CDN. If browser vendors decided which JS frameworks are bundled with the download and which ones are not, I'm not sure if that would help decentralisation!
But the better reason why these projects exist is for experimentation and iteration. It's not like everyone who uses React will switch to this over the holidays, but maybe in five years some of the ideas in here will become more mainstream, and a future version of React (or a different framework) may adopt them.