isn't it possible to store a cache locally and update it once every unit of time? This would radically lower the uptime requirement because the server now needs only to be online once every hour/day/week...

It's both uptime and trust, a local fallback will always be necessary. I could run a cache, but it would also be a third party service with questionable uptime, it's just good to have something built into the software itself because it's such an important core function