Interesting observation. Repetition may be natural, people don't follow 1000 podcasts, but maybe one or two. Therefore, every podcaster will almost be expected by their audience to cover many/most aspects of the topic. In fact, a podcast will largely do their own learning by having conversations with the same experts that already appeared on the 1000 other podcasts, i.e. the podcast essentially documents the podcaster's own learning journey.

I agree that this does not necessarily contribute to existing knowledge, however it creates a kind of learning community. Almost nobody wants to listen to old podcast episodes from a few years back, and they also want to interact with the podcaster on social media. So, it won't push the state of the art, but it will onboard new people who could one day contribute to it.