The Cache API is a standard taken from browsers. In the browser, cache.delete obviously only deletes that browser's cache, not all other browsers in the world. You could certainly argue that a global purge would be more useful in Workers, but it would be inconsistent with the standard API behavior, and also would be extraordinarily expensive. Code designed to use the standard cache API would end up being much more expensive than expected.
With all that said, we (Workers team) do generally feel in retrospect that the Cache API was not a good fit for our platform. We really wanted to follow standards, but this standard in this case is too specific to browsers and as a result does not work well for typical use cases in Cloudflare Workers. We'd like to replace it with something better.