The reason is to connect them together in one experiential flow. That was the purpose of hypermedia, the interconnection of platforms and data sources.

People aren't doing that, I realize it, but maybe we should fix that instead of abandon the dream.

All of those things center around the same set of technologies. Browsers today even come bundled with developer tools which still include HTML editors. But it's better for there to be two separate applications that both use separate copies of the same rendering engines and UI chrome? We might as well just make everything in Electron

Also, it's not like browsers are any less complicated now. They support things like 3D rendering and even MIDI device interfacing which are arguably more complicated for less benefit than supporting alternate protocols and technologies