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"
A content management system that doesn’t produce cacheable html isn’t scalable and one that does is. It’s not the job of the content management system to serve the cached pages. That’s what CDNs, browsers, and caching layers in general are for.