The answer is clear: It takes an effort to publish something on the internet. Companies and other organisations disappear and their web presence disappears with them. It takes an extra effort to create a copy. The dark web - the parts of the web that aren't easily accessible because they consist of databases, paid access areas or perhaps simply facebook posts not published as "public" - remains hidden and inaccessible to preservation efforts.

There's also the problem of consuming old internet content. Do you still have a gopher client? Will old web pages look the same in your browser as they did in 1995? Flash will no longer work (hooray?).

On the other hand books, photographs and magazines stick around until you throw them away and we have public institutions that are dedicated to their preservation.