In short, the cache date shown by Google, for many sites, shows dates over a month old. Typically the dates are fresher than that. For normal sites, the date for many is dated back to the October 20th range.
For example, this is my corporate site:
It shows October 21st.
But here is this site:
It shows today, as early as this morning.
Clearly, this site is updated way more often than my corporate site, but it isn't common for the cache date to be so old.
Google's John Mueller did respond to one question about this on Twitter basically saying don't worry about it. John wrote, "The cached date is separate of crawling & indexing, so I wouldn't use that as a metric."
But why is the cache date so dated?
Google has said in the past they do not update the cache on each crawl.