Paper doesn't need a battery
A printed book asks nothing of you but light and eyes. A web page, done well, can ask for almost as little.
Most of what a browser renders today would still make sense to someone reading it in fifty years, provided the markup stayed plain. Fonts can be swapped, stylesheets rewritten, but the words themselves don't rot.
That's not true of most formats built in the last decade. Proprietary document types, JavaScript-rendered content, apps that phone home to a server that no longer exists — all of it depends on something outside the file itself staying alive. A paragraph of HTML depends on nothing but a browser, and browsers are stubbornly backward compatible.
That's the standard this site tries to hold itself to.