Ask HN: Why don't Digg, Reddit show a cached copy of a submitted site?

The title says it all, there still are a lot of sites that go down after being slashdotted.

So why social news submitting sites don't just show a cached page of the site? (Also, if they feel like it they would be able to "justify" showing their header with ads above the site in a frame.)

How comes this problem wasn't addressed? Or was it?

Maybe this wouldn't help for AJAX sites (but it won't break them aswell), but for the rest of the static content i am pretty sure it will

Do you think this might be a startup idea? Perhaps it can be enhanced with a "show a page how it was 5 days ago, 3 months ago, etc" so if you suspect the page might change you can show it "freezed" without having to do ugly things like screenshoting it and submitting to tinypic.com