Parsing and processing several megabytes of JS for each page you visit can get expensive quickly. https://tonsky.me/blog/js-bloat/

Yes but isn’t this the beauty of decentralized protocols? The cost is spread amongst all, it doesn’t require one entity to shoulder the cost for everyone.

It’s not really decentralized in that way. If you’re loading a bunch of pages in your mobile browser, your device is paying the power consumption cost of deserializing, parsing, and running all that JS. It doesn’t save on that because some other user also did it.