Almost all graphQL clients implement a custom caching layer (for example using indexeddb in the browser) that can cache resources with different timelines, also when they’re returned in a single response.
Yeah, but that's client-side caching, which doesn't get you much in terms of scalability. Scalability comes from transparent backend reverse-proxy caching — Varnish and the like. You can cache the resources that go into a GraphQL response, but you still have to build the GraphQL response each time — and that can become a problem, as various should-be-trivial parts of each request's backend lifecycle start living long enough during high concurrency request loads to destroy request-queue clearance rates.