Caching is one facet of scaling (albeit a large one). It's not necessarily the job of a CMS to serve cached pages but there's a bunch of other considerations in

Caching is one facet of scaling (albeit a large one). It's not necessarily the job of a CMS to serve cached pages but there's a bunch of other considerations in scaling, particular where database writes are concerned; something that will be relevant in a system where users post content. Also, see the final paragraph of the comment above for discussion of the extent to which WP "produces cachable html"