Then ...

did you see a search result come from your previous search results :)
Google Desktop search results are generated as web pages while indexing web pages/histroies is part of Google Desktop features.
Caching results will cause recurisive indexing which may blow up all your hard disk space for nothing.
As a matter of fact, all search results are not cached and expired immediately in a web browser. Most web browsers keep history of your visited URLs but that alone only exposes the keywords you searched not the results.
We shouldn't blame a tool being so powerful :) Any powerful or not-so-powerful tools can be misused.

btw, I don't work for Google :) otherwise I'd be too busy counting how much money I've made from my options than reading/posting here.