Let's say I do a google search for "how to frobitz a widget" and the top result (because as you say it's in Google's cache) points me to a post on /r/WidgetFrobitzing.
I then click through and find that the post is deleted or has been changed to say "lol Spez sucks use Lemmy" or whatever. I'll almost certainly close that tab and go back to google to find another link. That deprives Reddit of clicks through its ads, of time spent on site, and it also means that user is less likely to follow links to Reddit in future as they will know they're not as useful as Google thought they were.