The short answer is that these RSS feeds are cached and are on a reload timer. Depending on the specific feed it will reload anywhere from 5 minutes to 1 hour.
This is done for 2 main reasons:
1. Produces the fastest in-app experience (the main reason)
2. Appropriate amount of communication with their servers. (This is also why they cache info as well.)
It's important to note that your service provider (Flickr, Vimeo, YouTube, etc.) likely caches this information as well, so there might be a cache reload timer on their part as well.