this content sometimes disappears from view
I've noticed that a fair number of the "official" reviews of films given on IMdb now point to parked domains.
The problem is that while big companies often keep their content around, smaller outfits have hosting to pay, domains to register, and when something runs to it's end then everything lapses and all of that content vanishes. Moreso if it was, say, a hobby site and the site maintainer went on to do something else, got married, whatever, and no longer has the time or inclination to continue. I also think a number of smaller sites got abandoned as simply having a page on Facebook was so much simpler (and cheaper) than dealing with HTML and hosting and uploading and...
In the case of big companies, it could be mergers or cutbacks or simply abandoning something seen as generating loss. A large number (at the time) of sites vanished when Geocities shut down (Yahoo! bought it, screwed up, then eventually Googled it into a shallow grave).
Ultimately, keeping stuff on the web costs somebody, somewhere, money. When the money goes away, so does all that data.