As you know, Google spiders, aka GoogleBot, will crawl sites like this hundreds of times per day. Google obviously won't update the cache result of this site hundreds of times per day. But on more static, less dynamic web sites, Google will crawl less often, sometimes once or less per day. You still can't expect Google to update the cache of your site on each crawl.
Google's JohnMu clarified this in a Google Webmaster Help thread saying:
In general, we do not always update the cached page every time that we crawl a page. Especially when the page does not significantly change, we may opt to just keeping the old date on it.
Technically, Google likely can update the cache more often but it is a matter of resources and utility. Personally the reason I use the cache, in most cases, is to see older content that might have been deleted. I personally like a few day delay in the Google Cache. Of course, most webmasters and SEOs want a more real time update of the Cache to better test their changes and such.
Forum discussion at Google Webmaster Help.