One of the biggest reasons is due to streaming media and rotating advertisements. If all that constantly changing, media-rich content was saved... forget about it!
The idea behind caching web pages was to speed delivery of content - if something didn't have to be re-fetched across the internet, your page opens sooner. But as website design became more and more dynamic, sites prefer to serve-up the current version of the site, not the version that was current two weeks ago.