The 'big sites' mostly use ASP, PHP, CFM or any other type of dynamic content.

As the pages are generated 'on the fly' and differently each time, your browser will recognize them as 'new' and load them again from the server.

Your Meta-Tag is principally OK, but you only need it for static HTML-pages.
And I personally found, that it won't always work. This seems to depend on the browser (??) or your provider's cache.

Wolfgang