Page Speed Testers Saying I Should Cache — I am, I am!

You’re mixing up two unrelated “caching” concepts.

WP Super Cache caches pages on the server, allowing the server software to send the same page quickly to a different visitor in the future.

The caching that webpagetest.org recommends is “browser caching” on visitor computers, which is different. It consists of setting “HTTP response headers” that tell a browser it’s okay to save a copy of (e.g.) images that change infrequently. This allows the cached copy already on the visitor’s computer to be displayed quickly if that person views the same page in the future, without needing to fetch the identical image from the server again.

The latter browser caching is unrelated to WP Super Cache. Searching Google for “browser caching expires” will show you some useful information about it.