It was a real need given how almost all sites use large JavaScript deps. However, any hopes of sharing those were destroyed by adtech people timing resource downloads to track people.
They are cached independently these days to avoid privacy issues. So if websites A and B both use the same JavaScript dependency from a public CDN and you visit them both, you will download the JavaScript dependency twice, even if you have it cached from your visit to the first website.
Who knows. Maybe the browser would be a more prevalent gaming platform if it could be assumed that loading a multi gigabyte game engine is no big deal, because everyone had one already cached.
A lot of unity games could easily be web games, but aren't because of many roadblocks. I believe this is one of them.